0001]The present invention relates to a stainless steel, more particularly, it relates to an austenitic stainless steel.
BACKGROUND
[0002]Recently, in place of fossil fuels, practical research has been actively transport devices that utilize hydrogen as energy. For example, a fuel cell vehicle travels hydrogen as fuel, and the development of hydrogen stations for supplying hydrogen to a fuel cell vehicle has been promoted.
[0003]
　When using stainless steel to a hydrogen station, stainless steel is used in high-pressure hydrogen gas environment. Therefore, the stainless steel used in the hydrogen station applications, is superior strength requirements.
[0004]
　WO 2012/132992 (Patent Document 1), WO 2004/083476 (Patent Document 2), International Publication No. 2004/083477 (Patent Document 3), and WO 2004/111285 (Patent Document 4) is used in a high-pressure hydrogen environment, we propose a stainless steel having a high strength.
[0005]
　High-pressure hydrogen gas austenitic stainless steel disclosed in Patent Document 1 by mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more and less than 7%, Cr: 15 ~ 30 % , Ni: 10% or more and less than 17%, Al: 0.10% or less, N: 0.10 ~ 0.50%, and V: 0.01 ~ 1.0% and Nb: 0.01 ~ 0.50 % and containing at least one of, the balance being Fe and impurities, P is 0.0050% among the impurities less, S is at 0.050% or less, a tensile strength of at least 800 MPa, grain size number ( in ASTM E 112) is No. 8 or more, a maximum diameter of 0.4 pieces / [mu] m the alloy carbonitrides of 50 ~ 1000 nm in cross-section observation 2 contains more.
[0006]
　Hydrogen gas stainless steel disclosed in Patent Document 2, in mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 3 ~ 30%, Cr: 22% to beyond 30% , Ni: 17 ~ 30%, V: 0.001 ~ 1.0%, N: 0.10 ~ 0.50%, and Al: contains 0.10% or less, the balance being Fe and impurities, P 0.030% of the impurity or less, S 0.005% or less, Ti, Zr and Hf is not more than 0.01%, respectively, and, Cr, the content of Mn and N are 5Cr + 3.4Mn ≦ 500 N meet.
[0007]
　High-pressure hydrogen gas stainless steel disclosed in Patent Document 3, by mass%, C: 0.04% or less, Si: 1.0% or less, Mn: 7 ~ 30%, Cr: 15 ~ 22%, Ni : 5 ~ 20%, V: 0.001 ~ 1.0%, N: 0.20 ~ 0.50% and Al: contains 0.10% or less, the balance being Fe and impurities, the impurities in P is 0.030% or less, S 0.005% or less, Ti, and the Zr and Hf is 0.01% or less, respectively, satisfy 2.5Cr + 3.4Mn ≦ 300N.
[0008]
　Patent Document 4 to the disclosed hydrogen gas austenitic stainless steel, by mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 0.01 ~ 30%, P: 0.040% less, S: 0.01% or less, Cr: 15 ~ 30%, Ni: 5.0 ~ 30%, sol. Al: 0.10% or less, N: 0.001 to 0.30% has a chemical composition the balance being Fe and impurities, X-rays integral of cross-section along the direction perpendicular to the processing direction intensity I (111) is not more than 5 times the random orientation, X-rays integrated intensity I (220) of the cross section along the working direction / I (111) containing tissue is ≦ 10.
CITATION
Patent Document
[0009]
Patent Document 1: WO 2012/132992 Patent
Patent Document 2: WO 2004/083476
Patent Document 3: WO 2004/083477 Patent
Patent Document 4: WO 2004/111285
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　Incidentally, in the stainless steel used in the hydrogen station applications, not only excellent strength, suppression of variation in the strength is also required. Stainless steel disclosed in Patent Documents 1 to 4 described above has a strength of at least 700MPa even after performing solution treatment, stainless steel of Patent Document 4, carried a solution treatment and cold working by having a high strength. However, discussion of intensity variations in these documents is not performed. Even stainless steel described in Patent Documents 1 to 4 described above, the variation of the intensity is large, there may not stable high strength can be obtained.
[0011]
　An object of the present invention is to provide an austenitic stainless steel having a stable high strength over the steel length.
Means for Solving the Problems
[0012]
　Austenitic stainless steel according to the present embodiment, by mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 3 ~ 8%, P: 0.05% or less, S: 0.03 % or less, Ni: 10 ~ 20%, Cr: 15 ~ 30%, N: 0.20 ~ 0.70%, Mo: 0 ~ 5.0%, V: 0 ~ 0.5%, and, Nb: contains 0 to 0.5% has a chemical composition the balance being Fe and impurities, ASTM grain size number conforming to E 112 is 6.0 or more. The tensile strength is at least 800 MPa, the difference between the maximum value and the minimum value of the tensile strength is less than 50 MPa. The number of the alloy carbonitrides circle equivalent diameter in the steel is more than 1000nm is 10 / mm 2 at least.
Effect of the invention
[0013]
　Austenitic stainless steel according to the present embodiment has a stable high strength over the steel length.
DESCRIPTION OF THE INVENTION
[0014]
　The present inventors have found that high-strength austenitic stainless steel, and research and investigated the intensity variation in steel full length, give the following findings.
[0015]
　As a method of increasing the (A) intensity, there is a solid solution strengthening and grain refining by N. The austenitic stainless steel of the present embodiment, increasing the strength by solid-solution strengthening contain from .20 to 0.70% of N. If refining crystal grains further strength increases.
[0016]
　(B) intensity variations in the steel full length, due to the grain size. As the variation of the crystal grain size in the steel is small, it can be reduced intensity variation. Specifically, it is grain size number based on ASTM E 112 6.0 or more, the difference between the maximum value and the minimum value of the grain size number of the steel Length (hereinafter, referred to as grain size difference DerutaGS) is 1.5 or less in some cases, the difference between the maximum value and the minimum value of the tensile strength in the steel Length (hereinafter, referred to as intensity difference .DELTA.TS) becomes less 50 MPa, it can be sufficiently suppressed intensity variations.
[0017]
　(C) in order to suppress the variations in strength, it is effective to suppress the temperature change of the material during the hot working. The variation of the crystal grain size is most prominently formed during hot working. Among the material, at a temperature lower portions and the temperature is high portion, the introduction amount of strain is different. Different amount of introduced distortion, grain refining degree upon recrystallization or different. Therefore, variations in the grain size becomes large. Accordingly, at the time of hot working, the temperature change of the material is preferably small.
[0018]
　Specifically, of the material, the first temperature during processing completion of the finish portion of the hot working (hereinafter, referred to as initial temperature) and, finally hot temperature during processing completion of the processing is completed portion (hereinafter , equal to or less than 100 ° C. is a difference (temperature difference [Delta] T) between that end temperature), the grain size difference ΔGS can be suppressed to 1.5 or less. As a result, it is possible to suppress the intensity difference ΔTS below 50 MPa.
[0019]
　(D) was carried out heat treatment on steel, if precipitation of coarse alloy carbonitrides, further increases the strength of the steel material by precipitation hardening. Grain size number of the steel is not less than 6.0, in the steel, the alloy carbonitrides equivalent circle diameter exceeding 1000 nm (hereinafter, coarse referred alloy carbonitrides) number of 10 pieces / mm 2 if more , or more of tensile strength of 800MPa can be obtained. By carrying out the heat treatment temperature of the heat treatment it is less than 930 ℃ ~ 1000 ℃, coarse alloy carbonitrides are 10 / mm 2 or more is obtained.
[0020]
　Here, the alloy carbonitrides, containing Cr, V, Nb, Mo, W, and Ta or the like as a main component, Cr 2 N, Z Sosoku Chi Cr (Nb, V) (C , N), and, MX-type (M: Cr, V, Nb , Mo, W, Ta , etc., X: C, N) means. The "main component" means that at least 40% by mass%. Further, the alloy carbonitrides of the present invention, the content of C (carbon) may ultimately less, i.e., wrapping the case of nitrides. Alloy carbonitrides of the present invention also comprises carbide.
[0021]
　Austenitic stainless steel of the present embodiment has been completed based on the above findings, by mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 3 ~ 8%, P: 0.05 % or less, S: 0.03% or less, Ni: 10 ~ 20%, Cr: 15 ~ 30%, N: 0.20 ~ 0.70%, Mo: 0 ~ 5.0%, V: 0 ~ 0 .5%, and, Nb: 0 contained to 0.5%, has a chemical composition the balance being Fe and impurities, ASTM grain size number conforming to E 112 is 6.0 or more. The tensile strength is at least 800 MPa, the difference between the maximum value and the minimum value of the tensile strength is less than 50 MPa. The number of the alloy carbonitrides circle equivalent diameter in the steel is more than 1000nm is 10 / mm 2 at least.
[0022]
　The chemical composition, by mass%, Mo: 1.5 ~ 5.0%, V: 0.1 ~ 0.5%, and, Nb: is selected from the group consisting of from 0.1 to 0.5% it may contain one or two or more.
[0023]
　In the above austenitic stainless steel, the difference between the maximum value and the minimum value of the grain size number is 1.5 or less.
[0024]
　The austenitic stainless steel, for example, a steel pipe, steel bar or wire rod.
[0025]
　It described in detail below austenitic stainless steel of the present embodiment. "%" Related elements, unless otherwise specified, it means mass%.
[0026]
　[Chemical composition]
　The chemical composition of the austenitic stainless steel of the present embodiment contains the following elements.
[0027]
　C: 0.10% or less
　carbon (C) is inevitably contained. C stabilizes the austenite is difficult fcc structure cause hydrogen embrittlement. C is further combined with Cr or the like, increasing the strength of steel by precipitation strengthening. However, if C content is too high, carbides are precipitated at grain boundaries to decrease the toughness of the steel. Therefore, C content is 0.10% or less. The preferable upper limit of C content is 0.08%, more preferably 0.06%. Moreover, the preferable lower limit of C content in order to stabilize the austenite is 0.005%.
[0028]
　Si: 1.0% or less
　silicon (Si) forms the intermetallic compound in combination with Ni and Cr. Si further promote the growth of the intermetallic compounds such as sigma phase (sigma phase). These intermetallic compound decreases the hot workability of steel. Therefore, Si content is 1.0% or less. The preferable upper limit of the Si content is 0.8%. From the viewpoint of deoxidation of the steel, the preferred lower limit of the Si content is 0.2%.
[0029]
　Mn: 3 ~ 8%
　manganese (Mn) is to stabilize the austenite, suppresses the generation of high susceptibility to hydrogen embrittlement martensite. Mn further forms MnS by combining with S, enhance the machinability of the steel. If the Mn content is too low, not the effect. On the other hand, if the Mn content is too high, decrease the ductility and hot workability of the steel. Therefore, Mn content is 3-8%. The preferable lower limit of the Mn content is 4.0%, more preferably from 5.0%. The preferable upper limit of the Mn content is 6.0%, more preferably from 5.9%.
[0030]
　P: 0.05% or less
　Phosphorus (P) is an impurity. P decreases the hot workability and toughness of the steel. Accordingly, P content is 0.05% or less. The preferable upper limit of the P content is 0.045%, more preferably 0.035%, still more preferably 0.020%. P content is preferably as small as possible.
[0031]
　S: 0.03% or less
　Sulfur (S) forms of MnS by combining with Mn, increasing the machinability of the steel. However, if the S content is too high, toughness of the steel is lowered. Thus, S content is 0.03% or less. The preferable upper limit of the S content is 0.02%, more preferably 0.01%. S content is preferably as small as possible.
[0032]
　Ni: 10 ~ 20%
　nickel (Ni) is to stabilize the austenite. Ni is further enhanced ductility and toughness of the steel. If the Ni content is too low, not the effect. On the other hand, if the Ni content is too high, the effect is saturated, the production cost is high. Therefore, Ni content is 10-20%. A preferable lower limit of Ni content is 11.5%, more preferably 12.0%. The preferable upper limit of the Ni content is 13.5%, still more preferably 13.4%.
[0033]
　Cr: 15 ~ 30%
　Chromium (Cr) enhances the corrosion resistance of steel. Cr is further combined with N Cr by heat treatment 2 to form an alloy carbonitrides such as N, increasing the strength of steel by precipitation strengthening. If the Cr content is too low, not the effect. On the other hand, if the Cr content is too high, M 23 C 6 type carbide is produced, which decreases ductility and toughness of the steel. Therefore, Cr content is 15 to 30%. A preferable lower limit of Cr content is 20.5%, more preferably 21.0%. The preferable upper limit of the Cr content is 23.5%, still more preferably 23.4%.
[0034]
　N: 0.20 ~ 0.70%
　nitrogen (N) is to stabilize the austenite. N further enhance the strength of steel by solid solution strengthening. N further, Cr combines with Cr by heat treatment 2 to form an alloy carbonitrides such as N, increasing the strength of steel by precipitation strengthening. If the N content is too low, not the effect. On the other hand, if the N content is too high, toughness of the steel is lowered. Therefore, N content is 0.20 to 0.70 percent. The preferable lower limit of the N content is 0.21%, more preferably from 0.22%. The preferable upper limit of the N content is 0.40%, more preferably 0.35%.
[0035]
　The remainder of the chemical composition of the austenitic stainless steel according to this embodiment, consists of Fe and impurities. Here, the impurity, when the industrial production of austenitic stainless steel, ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, adverse effect on the austenitic stainless steel of this embodiment It means what is allowed in a range which does not give.
[0036]
　[For any element]
　Austenitic stainless steel according to this embodiment further, in place of part of Fe, Mo, V, and may contain one or more members selected from the group consisting of Nb. Both of these elements increases the strength of steel.
[0037]
　Mo: 0 ~ 5.0%
　of molybdenum (Mo) is an optional element and may not be contained. If contained, Mo is solid solution strengthening austenite. Mo further enhances the corrosion resistance of steel. However, if Mo content is too high, the intermetallic compound is easily precipitated, which lowers the ductility and toughness of the steel. Therefore, Mo content is 0 to 5.0%. A preferable lower limit of Mo content is 1.5%, more preferably from 1.9%. The preferable upper limit of the Mo content is 3.0%, more preferably from 2.9%.
[0038]
　V: 0 ~ 0.5%
　vanadium (V) are optional elements may not be contained. If contained, V is generated carbides, increasing the strength of steel. However, if the V content is too high, the effect is saturated, the production cost is high. Therefore, V content is 0 to 0.5%. The preferable lower limit of V content is 0.1%, more preferably 0.12%. The preferable upper limit of the V content is 0.3%, more preferably 0.28%.
[0039]
　Nb: 0 ~ 0.5%
　niobium (Nb) is an optional element and may not be contained. If contained, Nb generates carbides, increasing the strength of steel. However, if the Nb content is too high, the effect is saturated, the production cost is high. Therefore, Nb content is 0 to 0.5%. The preferable lower limit of Nb content is 0.1%, more preferably 0.12%. The preferable upper limit of Nb content is 0.3%, more preferably 0.28%.
[0040]
　Strength and intensity difference .DELTA.TS]
　In austenitic stainless steel of the present embodiment, the tensile strength of not less than 800 MPa, and the difference between the maximum and minimum values of tensile strength (hereinafter, referred to as intensity difference .DELTA.TS) is below 50MPa is there. Accordingly, austenitic stainless steel material of this embodiment has a stable high strength over the steel length. The strength and intensity difference ΔTS, for example, can be realized in the following tissues.
[0041]
　[Grain size]
　In austenitic stainless steel of the present embodiment, the grain size number defined by ASTM E 112 is 6.0 or more. Grain size number is measured according to ASTM E 112. If the grain size number is less than 6.0, the strength is lowered. If the grain size number is 6.0 or more, high strength is obtained at the austenitic stainless steel of the aforementioned chemical composition. Specifically, necessary for austenitic stainless steel of the present embodiment, tensile strength of at least 800MPa can be obtained.
[0042]
　Grain size number is determined by the following method. To prepare a test piece for microscopic observation from a central portion in the longitudinal direction perpendicular to the cross section of the austenitic stainless steel. Of the surface of the test piece, using surface (called observation plane) corresponding to the cross section, and carrying out the microscopic test method for grain size defined in ASTM E 112, to evaluate the grain size number. Specifically, after mechanically polishing the observation surface, well-known etchant (Guriserejia, curling reagent or marble reagents) corrosion with, thereby revealing the grain boundaries of the observation plane. In 10 field on the corroded surface, obtaining the grain size number of each field. Area of each field of view, about 10.2 mm 2 is. By comparison with defined grain size standard view to ASTM E 112, to evaluate the grain size number in each field. The average crystal grain size number of each field is defined as the grain size number of austenite stainless steel material of the present embodiment.
[0043]
　[Grain size difference DerutaGS]
　Further, in austenitic stainless steel of the present embodiment, among the austenitic stainless steel total length, the difference between the maximum value and the minimum value of the measured grain size number in any of a plurality of portions (grain size difference DerutaGS that) is less than or equal to 1.5. If the grain size difference ΔGS exceeds 1.5, the difference between the maximum value and the minimum value of the tensile strength measured at a plurality of portions of the steel (intensity difference .DELTA.TS) exceeds 50 MPa, large variations in strength of the steel material full length Become. If the grain size difference ΔGS is 1.5 or less, the intensity difference ΔTS becomes less 50 MPa, the strength variation in steel full length is suppressed. Therefore, austenitic stainless steel material of this embodiment has a stable high strength.
[0044]
　Grain size difference ΔGS is measured by the following method. In the total length of the austenitic stainless steel, a plurality of any portion of the longitudinal direction to prepare a test piece for above the same microscope. With each test piece, similarly to the above, we conducted microscopic test method for grain size defined in ASTM E 112, determine the grain size number. Among the obtained grain size number, select the maximum value and the minimum value, it defines the difference between the maximum value and the minimum value and the grain size difference DerutaGS. When austenitic stainless steels are steel, steel bar, wire rod and the like, hot working direction (rolling direction, the direction of extrusion, etc.) A test piece was sampled from the steel both ends in (top part and bottom part), grain size difference ΔGS the seek. Here, the top portion is defined from steel tip portion within the range of 200mm toward the center, the bottom portion and the portions within the range of 200mm of steel rear toward the center.
[0045]
　Grain size difference ΔGS is preferably small. The preferable upper limit of the grain size difference ΔGS is 1.3, more preferably 1.0.
[0046]
　[Alloy carbonitrides]
　it was carried out heat treatment on the steel material, if precipitation of coarse alloy carbonitrides, the strength of the steel material is increased by precipitation strengthening.
[0047]
　Alloy carbonitrides contains Cr, V, Nb, Mo, W, and Ta or the like as a main component, Cr 2 N, Z Sosoku Chi Cr (Nb, V) (C , N), and, MX-type (M : including C, and N): Cr, V, Nb , Mo, W, Ta , etc., X. Also, carbon nitride in the present invention includes the case when the content of C (carbon) is ultimately less, i.e., a nitride. Carbonitrides in the present invention includes carbides.
[0048]
　In this embodiment, the number of the alloy carbonitrides circle equivalent diameter in the steel is greater than 1000 nm (coarse alloy carbonitrides) is 10 pieces / mm 2 at least. In this case, by precipitation hardening, high tensile strength can be obtained. Because they may decrease the toughness of the steel if coarse alloy carbonitrides is too large, the preferred upper limit of the number of coarse alloy carbonitrides in the steel, 1.5 × 10 5 cells / mm 2 is. By carrying out the heat treatment temperature of the heat treatment it is less than 930 ℃ ~ 1000 ℃, 10 pieces / mm 2 or more coarse alloy carbonitrides are obtained.
[0049]
　[Number measuring method of coarse alloy carbonitrides]
　number of coarse alloy carbonitrides is defined as follows. Obtaining a sample including the central portion of the cross section perpendicular to the longitudinal direction of the austenitic stainless steel (observation region with a radius 10mm around the steel central axis). The observation region of the sample is mirror polished. Thereafter, in any 10 fields of the observation area (200μm × 200μm), using a scanning electron microscope (SEM) with energy dispersive X-ray spectrometer (EDS), in the precipitates and inclusions of the field from identifying the alloy carbonitrides. The circle equivalent diameter of each alloy carbides identified in each field determined by image analysis. The circle-equivalent diameter refers to the diameter (nm) when converted to the area of the alloy carbides in the field of view in the circle. Circle equivalent diameter measured the number of alloy carbonitrides of greater than 1000 nm (coarse alloy carbonitrides). The average value of the number of the obtained coarse alloy carbonitrides at 10 fields respectively, herein, the number of coarse alloy carbonitrides (pieces / mm 2 defined as).
[0050]
　[Manufacturing Method]
　An example of a manufacturing method of austenitic stainless steel material of the present embodiment will be described. This manufacturing method includes a cooling step of cooling preparing step of preparing a material, hot working step of producing the intermediate member by carrying out the hot working against material, hot working the intermediate material, and, optionally Te, comprising a heat treatment step of performing heat treatment on the cooled intermediate member. Hereinafter, a manufacturing method will be described.
[0051]
　[Preparation Step]
　to produce a molten steel having the chemical composition described above. Relative produced molten steel, carrying out the known degassing treatment if necessary. From molten steel was carried out degassing treatment, to produce the material. Method for producing a material for example, a continuous casting method. By continuous casting, to produce continuously cast material (Material). Continuous casting material is for example, slabs, blooms and billets and the like. Molten steel may be ingot by ingot-making method.
[0052]
　[Hot working step]
　Material to hot working by known methods (continuous cast material or ingots), to produce an intermediate material of austenitic stainless steel. Intermediate material is, for example, steel pipes, steel bars and wire rods and the like. Intermediate material, for example, is produced by hot extrusion by Ugine Sejournet-Sejurune method.
[0053]
　The heating temperature and reduction of area in hot working process is as follows.
[0054]
　Heating temperature: 1160 ° C. or less
　if the heating temperature is too high, crystal grains are coarsened, the crystal grain size number of the steel structure is less than 6.0. Therefore, the heating temperature is below 1160 ° C.. Preferred upper limit of the heating temperature is 1100 ° C..
[0055]
　The lower limit of the heating temperature may be a known temperature. If the heating temperature is too low, even if heat treatment is performed below after hot working, coarse alloy carbonitrides is hardly generated. Thus, a preferred lower limit of the heating temperature is 1060 ° C..
[0056]
　Reduction of area: 70% super
　hot working before the cross-sectional area of the material A0 (mm 2 ), the final cross-sectional area of the material after hot working A1 (mm 2 when the), reduction of area RA (% ) is defined by equation (1).
　RA = (A0-A1) / A0 × 100 (1)
[0057]
　If the reduction of area is 70% or less, due to the lack of amount of strain introduced into the steel, the crystal grains are hardly miniaturized. If reduction of area of ​​70% or more, sufficient strain by hot working is introduced finer crystal grains, the crystal grain size number is 6.0 or more. A preferred lower limit of the reduction of area is 75%.
[0058]
　Temperature difference of the material during hot working [Delta] T: 100 ° C. or less
　in the hot working step, among the materials, the temperature during the first hot working the completion of the finish portion of the hot working (referred to as initial temperature), the end the difference between the hot working upon completion of the temperature of complete parts hot working (referred end temperature) (temperature difference [Delta] T) is 100 ° C. or less.
[0059]
　For example, piercing rolling, hot extrusion, to produce intermediate product was conducted hot rolling, the first to complete the hot working portion of the material is the top portion of the material, and finally completed the hot working portion that is the bottom part. Therefore, in this case, the initial temperature is the temperature of the hot working completion of the top portion, the end temperature is the temperature of the hot working completion of the bottom portion.
[0060]
　If it exceeds 100 ° C. is material temperature difference [Delta] T, the temperature variation in the steel overall length is too large. In this case, significantly different and the grain size of the crystal grain size and the bottom portion of the top portion, the grain size difference ΔGS exceeds 1.5. As a result, intensity difference ΔTS exceeding 50 MPa.
[0061]
　If the material temperature difference ΔT is at 100 ° C. or less, is suppressed variations in the grain size of the top portion and bottom portion, the grain size difference ΔGS is 1.5 or less. As a result, intensity difference ΔTS becomes less 50 MPa. The preferable upper limit of the temperature difference ΔT is 90 ° C., more preferably from 80 ° C..
[0062]
　[Cooling Step]
　The cooling step, cooling the hot intermediate product after processing at 0.10 ° C. / sec or higher. If the cooling rate is less than 0.10 ℃ / sec, σ phase is precipitated. σ phase decreases the corrosion resistance. To increase the corrosion resistance must suppress the generation of σ phase. If the cooling rate is less than 0.10 ° C. / sec is further crystal grains are coarsened strength of the steel is lowered. Therefore, the cooling rate is 0.10 ° C. / sec or higher.
[0063]
　The intermediate product after cooling, and carrying out the straightening may force the bending of the intermediate product. When carrying out straightening, for example, downstream of the cooling device, and / or, to dispose the straightening machine in-line or off-line bend upstream of the heating device.
[0064]
　With respect to the cooling or after bending the intermediate product after correction may be carried out descaling process. Descaling process, for example, be carried out by pickling or shot blasting. Descaling treatment is subjected to heating in earlier steps, performed in order to remove the oxide scale which is unavoidably formed on the surface of the intermediate product. Through the above process, the austenitic stainless steel of the present embodiment is manufactured.
[0065]
　[Heat treatment step]
　In the heat treatment step, the coarse alloy carbonitrides 10 / mm 2 to precipitate more. This further increases the tensile strength of the austenitic stainless steel. The heat treatment temperature is as follows.
[0066]
　Heat treatment temperature: 930 ℃ ~ 1000 ℃ less than
　is less than the heat treatment temperature is 930 ° C., without tissue austenite single phase is obtained, strength is lowered. Further, if the heat treatment temperature is lower than 930 ° C., sigma phase is generated, the corrosion resistance of the steel is lowered. On the other hand, if the heat treatment temperature is 1000 ° C. or higher, or coarse alloy carbonitrides in the steel is reduced, or will be completely dissolved, the number of coarse alloy carbonitrides 10 / mm 2 and less than Become. As a result, precipitation strengthening can not be obtained.
[0067]
　If the heat treatment temperature is lower than 930 ° C. ~ 1000 ° C., coarse alloy carbonitrides are precipitated, the number of coarse alloy carbonitrides 10 / mm 2 becomes higher. As a result, further increase the strength of the steel material by precipitation hardening. Further is less than the heat treatment temperature is 1000 ° C., coarse alloy carbonitrides are sufficiently precipitate stable 800MPa or more strength can be obtained in a range of less than 6.0-8.0 grain size number.
[0068]
　Even the heat treatment temperature is outside the above range, the grain size number is not less than 6.0, the number of coarse alloy carbonitrides in the steel 10 / mm 2 if more, high strength can be obtained, if the grain size difference ΔGS is 1.5 or less, it is possible to suppress the intensity difference ΔTS below 50 MPa.
[0069]
　It is not particularly limited holding time at the heat treatment temperature in the heat treatment but, for example at least 1 minute.
[0070]
　Manufacturing method according to the present embodiment, after the heat treatment step is carried out cold working may include a cold working step. However, there are cases where coarse alloy carbonitrides can not be obtained, a solution treatment after cold working process is not performed.
Example
[0071]
　Chemical composition of Table 1 was prepared molten steel having.
[0072]
[Table 1]

[0073]
　By using the molten steel, to produce an ingot of 3400kg. Hot working to to produce austenitic stainless steel rod (intermediate product) (diameter 45 ~ 75 mm × length 3000 mm) with respect to the ingot. The initial temperature at the time of hot working (hot temperature during extrusion completion of the top portion), (hot temperature during extrusion completion of the bottom portion) end temperature, and reduction of area RA (%) as shown in Table 2 Met.
[0074]
[Table 2]

[0075]
　The prepared raw tube was cooled at a cooling rate shown in Table 2. Further, with respect to the cooled hollow shell was carried bend the straightening and descaling process. Further, to produce a austenitic stainless steel (steel pipe) and heat treatment is performed at a heat treatment temperature shown in Table 2. The retention time was 45 minutes. In Test No. 18, heat treatment was not performed. The tensile strength (grain size) has a large influence of the processing completion temperature during hot working, the temperature is high top portion higher strength (grain size is small), the temperature is the bottom portion is low low intensity (grain size large) They tend to be. Therefore, the maximum value of the tensile strength and the minimum value was measured at the top portion and the bottom portion.
[0076]
　[Grain size number Measurement
　using a test piece taken from the top portion and bottom portion in the hot working of the steel material of each test number produced was performed grain size test based on ASTM E 112 described above. Samples were taken from the position (thick central portion) corresponding to the top portion and the bottom portion of each steel. Determined grain size number of the top portion and the bottom portion, further, to determine the grain size difference DerutaGS. The obtained grain size number and the grain size difference ΔGS shown in Table 2.
[0077]
　[Number Measurement of coarse alloy carbonitrides]
　were collected specimen from thick central portion of the steel of each test number. Using harvested specimens, the number of coarse alloy carbonitrides (pieces / mm by the above method 2 was determined).
[0078]
　[Tensile test]
　the top portion of the steel material of each test number, from the center of the bottom part, were taken round bar tensile test specimens. Round bar tensile test specimen includes a thick central portion of the steel material (steel), the parallel portion of the round bar test piece was parallel to the longitudinal direction of the steel material. The diameter of the parallel portion was 5 mm. Using a rod test piece, in conformity with JIS Z2241 (2011), normal temperature (25 ° C.), and a tensile test in the air, the top portion of each test number, the bottom portion tensile strength TS (MPa) I was asked. Furthermore, to determine the intensity difference .DELTA.TS (MPa) of each test number.
[0079]
　[Test Results]
　The test results are shown in Table 2.
[0080]
　Referring to Table 2, the chemical composition and manufacturing conditions of the steel with test Nos. 1 to 4 was appropriate. Therefore, the crystal grain size number is not less than 6.0, the grain size difference ΔGS was 1.5 or less. Furthermore, the number of coarse alloy carbonitrides 10 / mm 2 was over. Therefore, high tensile strength and more than 800 MPa, furthermore, the intensity difference ΔTS is less 50 MPa, stable and high strength over a steel entire length was obtained.
[0081]
　On the other hand, in Test No. 5-7, although the chemical composition was appropriate, the heating temperature of the hot working is too high. Therefore, the grain size number of the top portion and / or the bottom portion was less than 6.0. As a result, the strength of the steel is less than 800 MPa, the strength was low.
[0082]
　In Test No. 8, although the chemical composition was appropriate, the temperature difference ΔT at the time of hot working exceeds the 100 ° C., and reduction of area was less than 70%. Therefore, the grain size number is less than 6.0, the grain size difference ΔGS exceeds 1.5. As a result, the strength of the steel is less than 800 MPa, the strength was low. Furthermore, the intensity difference ΔTS exceeds 50 MPa, the strength variation is large.
[0083]
　In Test No. 9, although the chemical composition was appropriate, reduction of area during hot working was less than 70%. Therefore, the grain size number is less than 6.0. As a result, the tensile strength is less than 800MPa, the intensity was low.
[0084]
　In Test No. 10, although the chemical composition was appropriate, the cooling rate after hot working is less than 0.10 ° C. / sec. Therefore, the grain size number is less than 6.0. As a result, the strength of the steel is less than 800 MPa, the strength was low.
[0085]
　In Test No. 11, although the chemical composition was suitable, the heat treatment temperature after cooling was less than 930 ° C.. As a result, the strength of the steel is less than 800 MPa, the strength was low.
[0086]
　In Test No. 12, although the chemical composition was suitable, the heat treatment temperature after cooling is too 1200 ° C. and higher. Therefore, the number of coarse alloy carbonitrides 10 / mm 2 becomes less than the crystal grain size number is less than 6.0. As a result, the tensile strength is less than 800MPa.
[0087]
　In Test No. 13, N content was too low. As a result, the tensile strength is less than 800MPa.
[0088]
　In Test No. 14 and 15, although the chemical composition was suitable, the heat treatment temperature after cooling was 1000 ° C. or higher. Therefore, the number of coarse alloy carbonitrides 10 / mm 2 was below. As a result, the tensile strength is less than 800MPa.
[0089]
　In Test No. 16 and 17, although the chemical composition was appropriate, the steel temperature difference ΔT during hot working exceeds 100 ° C.. Therefore, the grain size difference ΔGS exceeds 1.5. As a result, intensity difference ΔTS exceeds 50 MPa, the strength variation is large.
[0090]
　In Test No. 18, it was not carried out the heat treatment. Therefore, there was no coarse alloy carbonitrides. As a result, the tensile strength is less than 800MPa.
[0091]
　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.

WE CLAIM

A austenitic stainless steel,
　by
　mass%, C: 0.10% or
　less, Si: 1.0% or
　less, Mn:
　3 ~ 8%, P: 0.05% or
　less, S: 0.03% or less , 　Ni:
　10 ~ 20%, Cr: 15 ~ 30%, 　N: 0.20 ~ 0.70%, Mo: 0 　~ 5.0%, V: 0 ~ 0.5%, 　and, Nb: 0 ~ containing 0.5%, it has a chemical composition the balance being Fe and 　impurities, grain size number conforming to ASTM E 112 is not less than 6.0, 　a tensile strength of not less than 800 MPa, 　the tensile strength the difference between the maximum value and the minimum value is not more than 50 MPa, 　the number of the alloy carbonitrides circle equivalent diameter in the steel is more than 1000nm is 10 / mm 2 at least, austenitic stainless steel.

[Requested item 2]
　A austenitic stainless steel of claim 1,
　wherein the chemical
　composition, Mo:
　1.5 ~ 5.0%, V: 0.1 ~ 0.5%,
　and, Nb: 0.1 ~ 0 one or containing two or more, austenitic stainless steel is selected from the group consisting of .5%.
[Requested item 3]
　A austenitic stainless steel according to claim 1 or claim 2,
　difference between the maximum value and the minimum value of the grain size number of 1.5 or less, austenitic stainless steel.
[Requested item 4]
　A austenitic stainless steel according to any one of claims 1 to 3,
　wherein the austenitic stainless steel is a steel pipe, steel bar or wire rod, austenitic stainless steel.