Technical field
[0001]
　The present invention, austenitic stainless steel and relates to a manufacturing method thereof, in particular, austenitic stainless steels excellent in naphthenic acid corrosion resistance and a method for producing the same.
BACKGROUND
[0002]
　Recently, thermal power boiler from tight energy demand, the construction of new oil refineries and petrochemical industrial plant is progressing. Then, in the austenitic stainless steel used in the heating furnace tube, etc., it has been required to have excellent corrosion resistance. In particular, has been rising more and more crude oil prices by the emerging economic growth, it has come to the use of poor quality crude oil will be considered at a low cost that has not been used up to now.
[0003]
　Under such technical background, for example, Patent Document 1, corrosion resistance superior chimney-flue and desulfurizing equipment for high alloy stainless steel is disclosed. In Patent Document 2, the heat exchanger of the heavy oil-fired boiler, flue, excellent sulfidation acid dew-point corrosion stainless steel for use in a portion corrosion by sulfuric acid liquid such as chimneys is a problem is disclosed.
[0004]
　Furthermore, Patent Document 3, heat exchangers used in thermal power generation and industrial boilers, flue, austenitic stainless steel is disclosed having excellent resistance to such a problem with sulfuric acid corrosion chimney . Then, Patent Document 4, high corrosion resistance, among others, have a high resistance to polythionic acid SCC, C immobilized element-containing austenitic stainless steels have been disclosed.
CITATION
Patent Document
[0005]
Patent Document 1: JP-A-2-170946 JP
Patent Document 2: JP-A 4-346638 Patent Publication
Patent Document 3: JP 2000-1755 JP
Patent Document 4: WO 2009/044802
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　When using the low-quality crude oil in the distillation column Equipment of petroleum refining plant, naphthenic acid corrosion is known to occur. In the future, it is not necessary to purify by raising the low-quality crude oil for use ratio arise, this problem of corrosion is expected to come actualized.
[0007]
　However, not fully consider the Patent Documents 1 to problems of naphthenic acid corrosion, as described above in 4, the development of the steel has been required to have excellent resistance to naphthenic acid corrosion.
[0008]
　The naphthenic acid corrosion, heretofore total acid (Total Acid Number: TAN) and flow rate have been said to be dominant. However, not necessarily a correlation seen in TAN and corrosion rate, the type and structure of acid, are believed to vary depending on the content. Thus, prediction of corrosion rate by the analysis of the corrosive environment is at present, it is very difficult. Therefore, work to review the material surface, it is necessary to perform component designs of materials excellent in naphthenic acid corrosion.
[0009]
　The present invention has been made to solve the above problems, and an object thereof is to provide excellent resistance to naphthenic acid corrosion resistance and economy and austenitic stainless steel and a manufacturing method that combines.
Means for Solving the Problems
[0010]
　The present inventors have made intensive studies on components E900 having excellent resistance to naphthenic acid corrosion, has led to obtain the following findings.
[0011]
　As an effective method for improving the resistance to naphthenic acid corrosion, a method of increasing the Mo content is known. However, if excessively contained the Mo, for example, toughness and performance other than corrosion resistance of the weld resistance and the like becomes to problem worse. Also, it is contained expensive Mo recklessly is not preferable because it becomes a cause of deteriorating the economical efficiency.
[0012]
　Therefore, Mo is to consider in detail the mechanism of improving the resistance to naphthenic acid corrosion, the surface layer of the naphthenic acid corrosion test after the test piece was investigated. As a result, there is formed a coating of MoS sulfides of Mo on the test piece surface layer, the coating was found to have contributed to the improvement of resistance to naphthenic acid corrosion.
[0013]
　However, the number of Mo is present as a precipitate, does not play the role of the material of the film, it was found to interfere with the production of film rather. That is, by simply increasing the total amount of Mo is insufficient, it is necessary to solid solution of Mo in the steel.
[0014]
　The present invention has been made with the above findings as a basis, austenitic stainless steel following the steel using the same, steel pipes, steel plates, steel bars, wire rods, forged steel, valves, fittings and welding material, as well as the austenitic stainless the method for producing steel and gist.
[0015]
　(1) chemical composition, in
　mass%, C: 0.015% or
　less, Si: 1.00% or
　less, Mn: 2.00% or
　less, P: 0.05% or
　less, S: 0.030% or less ,
　Cr: less than 16.0% or more%
　22.0,
　Ni: 11.0 ~ 16.0%, Mo: 2.5
　~ 5.0%, N: less than 0.07% or more 0.15%,
　Nb :
　0.20
　～ 0.50 Pasento, Al:
　0.005 ～ 0.040 Pasento, Sn: 0 ～ 0.080 Pasento,
　Zn: 0 ～ 0.0060 Pasento, Pb: 0 ～ 0.030 Pasento,
　the balance: is Fe and impurities,
　satisfies the following formula (i), austenitic stainless steel.
　Mo SS /Mo≧0.98 · · · (i)
　where, Mo in the formula represents the content of Mo contained in the steel (mass%), Mo SS is a solid solution in the steel Mo It represents the content (mass%).
[0016]
　(2) the chemical composition,
　further described below R is defined by (ii) Formula 1 value satisfies (iii) below formula, austenitic stainless steel according to the above (1).
　R 1 = 0.25Ni + 0.2Cr + (Mo × (Mo SS / Mo)) 1.45　　　· · ·
　(ii) R 1 ≧ 10.2 · · · (iii)
　where, (ii) each element symbol in the formula represents the content of each element contained in the steel (mass%), Mo SS represents Mo content in solid solution in the steel (mass%).
[0017]
　(3) the chemical composition, by
　mass%,
　Sn: 0.002 ~ 0.080%, Zn: 0.0002 ~ 0.0060%, and
　Pb: 0.0005 ~ 0.030%,
　is selected from contain one or more,
　further L value defined by the following (iv) expression is satisfied (v) below formula, austenitic stainless steel according to the above (1). = 7.6Sn
　L 0.18 + 9.5Pb 0.18 + 12.8Zn 0.2　　　· · · 　(iv) 1.8 ≦ L ≦ 13.9 · · · (v) 　where the in (iv) Formula element symbol represents the content of each element contained in the steel (mass%).

[0018]
　(4) the chemical composition, by
　mass%,
　Sn: 0.002 ~ 0.080%, Zn: 0.0002 ~ 0.0060%, and
　Pb: 0.0005 ~ 0.030%,
　is selected from contain one or more,
　further L value defined by the following (iv) expression is satisfied (v) below formula, austenitic stainless steel according to the above (2). = 7.6Sn
　L 0.18 + 9.5Pb 0.18 + 12.8Zn 0.2　　　· · · 　(iv) 1.8 ≦ L ≦ 13.9 · · · (v) 　where the in (iv) Formula element symbol represents the content of each element contained in the steel (mass%).

[0019]
　(5) the chemical composition,
　further R defined by the following (vi) expression 2 value satisfies the following (vii) expression, austenitic stainless steel according to the above (4).
　R 2 = R 1 + (L-5.1) / 3 · · ·
　(vi) R 2 ≧ 12.0 · · · (vii)
　Here, R in (vi) expression 1 in the above (ii) Formula a defined values, L is a value defined by (iv) above equation.
[0020]
　(6) Average grain size number inside the steel has a metal structure is 7.0 or more, austenitic stainless steel according to any one of (1) to (5).
[0021]
　(7) The austenitic stainless steel with austenitic stainless steel according to any one of (1) to (6).
[0022]
　(8) an austenitic stainless steel tube with austenitic stainless steel according to any one of (1) to (6).
[0023]
　(9) austenitic stainless steel with austenitic stainless steel according to any one of (1) to (6).
[0024]
　(10) (1) to (6) austenitic with austenitic stainless steel according to any one of up to Sutenresuboko.
[0025]
　(11) (1) to (6) to austenitic stainless wire with austenitic stainless steel according to any one of.
[0026]
　(12) austenitic stainless forged steel with austenitic stainless steel according to any one of (1) to (6).
[0027]
　(13) austenitic stainless valve using the austenitic stainless steel according to any one of (1) to (6).
[0028]
　(14) austenitic stainless joints using an austenitic stainless steel according to any one of (1) to (6).
[0029]
　(15) austenitic stainless welding material with austenitic stainless steel according to any one of (1) to (6).
[0030]
　(16) above (1) a steel having a chemical composition according to any one of up to (5), a heat treatment step of heating 8h over a temperature range of 1370 ° C. or less exceed 1260 ° C.,
　the finishing temperature is 960 - a hot working step of performing hot working to be in the range of 1150 ° C.,
　comprising a finishing heat treatment step of heating or 3min in the temperature range of 1000 ~ 1100 ° C., a manufacturing method of austenitic stainless steels.
[0031]
　(17) percent reduction of cross section further comprises a cold working step of performing cold working under conditions such that 30% or more, a manufacturing method of austenitic stainless steel according to the above (16).
Effect of the invention
[0032]
　According to the present invention without deteriorating the steel quality, it is possible to obtain a with compatibility and excellent naphthenic acid corrosion resistance and economy austenitic stainless steels. Austenitic stainless steel according to the present invention, for example, petroleum refining plant piping, furnaces, distillation columns are suitable for use column, among others components, a pump, as austenitic stainless steel such as a heat exchanger. For example, austenitic stainless steel is steel, steel sheet, steel bars, wire rods, forged steel, valves, fittings, and welding material or the like.
DESCRIPTION OF THE INVENTION
[0033]
　It will be described in detail below each requirement of the present invention.
[0034]
　(A) Chemical composition
　reasons for limiting each element are as follows. Incidentally, "%" for the content in the following description means "mass%".
[0035]
　C: 0.015% or less
　C, as well as has the effect of stabilizing the austenite phase, to form a fine intragranular carbide, an element which contributes to the improvement of high-temperature strength. Therefore, it is from the viewpoint of securing high-temperature strength to contain C in an amount commensurate with the amount of carbide forming elements, from the viewpoint of strengthening by precipitation of intragranular carbide. However, C is, the corrosion resistance, among others, from the viewpoint of securing resistant naphthenic acid corrosion resistance, in order to suppress sensitization due to precipitation of Cr carbides combined with Cr, the content thereof as much as possible it is desirable to reduce. It becomes excessive content of C, and particularly more than 0.015%, lead to significant degradation of corrosion resistance. Therefore, C content is 0.015% or less. C content is preferably 0.010% or less. On the other hand, when it is desired to obtain the aforementioned effects, C content is preferably 0.005% or more.
[0036]
　Si: 1.00% or less
　Si has a deoxidizing effect in the step of melting the austenitic stainless steels, also is an element effective for enhancing the oxidation resistance, steam oxidation resistance and the like. However, Si is because an element which stabilizes ferrite phase, the content thereof is excessive, in particular, when it exceeds 1.00%, decreases the stability of the austenite phase. Therefore, Si content is at most 1.00%. Si content is preferably at 0.80%, and more preferably 0.65% or less. On the other hand, when it is desired to obtain the aforementioned effects, Si content is preferably to 0.02% or more, more preferably 0.10% or more.
[0037]
　Mn: 2.00% or less
　Mn, as well as an element which stabilizes austenite phase, other suppression of hot working brittleness due to S, is an effective element for deoxidation during melting. However, when the content exceeds 2.00%, sigma and promotes the precipitation of intermetallic phases of equality, when used under high temperature environments, toughness and due to the deterioration of microstructural stability at high temperatures It leads to a decrease in ductility. Therefore, Mn content is at most 2.00%. Mn content is preferably not more than 1.50%. On the other hand, when it is desired to obtain the aforementioned effects, Mn content is preferably to 0.02% or more, more preferably 0.10% or more.
[0038]
　P: 0.05% or less
　P promotes intergranular corrosion, and to provide a reduction in grain boundary strength, deteriorating the resistance to naphthenic acid corrosion. Accordingly, P content is 0.05% or less. P content is preferably not more than 0.035%.
[0039]
　S: 0.030% or less
　S, like the P, to promote intergranular corrosion, and to provide a reduction in grain boundary strength, deteriorating the resistance to naphthenic acid corrosion. Thus, S content is 0.030% or less. S content is preferably not more than 0.025%.
[0040]
　Cr: less 16.0% than 22.0%
　Cr is an essential element for ensuring the oxidation resistance and corrosion resistance at high temperatures, in order to obtain this effect, necessary to contain more than 16.0% there is. However, the content thereof is excessive, in particular, not less than 22.0%, to lower the stability of the austenite phase at high temperatures, leading to reduction in creep strength. Therefore, the content of Cr is less than 22.0% or more 16.0%. Cr content is preferably 17.0% or more. Further, it is preferable Cr content is less 21.0 percent, and more preferably not more than 20.0%.
[0041]
　Ni: 11.0 ~ 16.0% Ni
　is an essential element for ensuring a stable austenitic microstructure, to ensure long tissue stability during use, essential in order to obtain the desired creep strength which is the element. To obtain the effect sufficiently, the balance of the above-described Cr content is important, considering the lower limit of the Cr content in the present invention, Ni content should be 11.0% or more . On the other hand, the Ni is an expensive element, causing an increase of the inclusion costs exceed 16.0%. Therefore, the Ni content is set to 11.0 to 16.0%. Ni content is preferably 11.8% or more is preferably not more than 14.3%.
[0042]
　Mo: 2.5 ~
　5.0% Mo, the improvement of high-temperature strength by solid solution in the matrix, an element which contributes to the improvement of creep strength at inter alia a high temperature. Further, Mo has also the effect of suppressing the grain boundary precipitation of Cr carbides. Furthermore, Mo is combined with S in the use environment to form a sulfide film, which contributes to the improvement of resistance to naphthenic acid corrosion. To obtain these effects, it is necessary to include Mo 2.5% or more. However, the stability of the austenite phase decreases the contrary creep strength for reducing the content of Mo increases. In particular, when the Mo content exceeds 5.0%, decrease in the creep strength is significantly. Therefore, Mo content is 2.5 to 5.0%. Mo content is preferably 2.8% or more is preferably not more than 4.5%.
[0043]
　As discussed above, the directly involved in the improvement of resistance to naphthenic acid corrosion is the Mo in solid solution. When Mo content present as a precipitate is excessive, not only the lack of solute Mo as a material for the sulphide coating, interfere with the formation of the coating. Thus, in addition to Mo content is within the above range, the proportion of solid solution amount to the total amount of Mo needs to satisfy the following formula (i).
　Mo SS /Mo≧0.98 · · · (i)
　wherein each element symbol in the formulas represents the content of each element contained in the steel (mass%), Mo SS is a solid solution in the steel and the Mo content are (% by mass) represents.
[0044]
　In addition to the proportion of solid solution relative to the entire quantity of Mo, by incorporating a well-balanced contents of Ni and Cr, it is possible to improve the economic resistant naphthenic acid corrosion. Therefore, in relation to the content of the amount of solid solution of Ni and Cr to the total amount of Mo, further R defined by the following (ii) Formula 1 is preferably the value satisfies the (iii) below formula.
　R 1 = 0.25Ni + 0.2Cr + (Mo × (Mo SS / Mo)) 1.45　　　· · ·
　(ii) R 1 ≧ 10.2 · · · (iii)
　where, (ii) each element symbol in the formula represents the content of each element contained in the steel (mass%), Mo SS represents Mo content in solid solution in the steel (mass%).
[0045]
　N: less 0.07% or more 0.15%
　N is an element which stabilizes austenite phase, precipitated as fine carbonitride in the grains as well as a solid solution in the matrix, to improve the creep strength it is an effective element. To obtain these effects sufficiently, it is necessary that the N content is 0.07% or more. However, when the content of 0.15% or more excess N, resistance to naphthenic acid corrosion by sensitization for Cr nitrides are formed in the grain boundary is degraded. Therefore, the content of N should be less than 0.07 to 0.15 percent. N content is preferably at 0.09% or more is preferably not more than 0.14%.
[0046]
　Nb: 0.20 ～ 0.50
　Pasento Nb is a C fixing element. By carbide Nb and C is bonded is precipitated in the grains, sensitizing is suppressed is suppressed precipitation of Cr carbides at grain boundaries, it becomes possible to ensure high corrosion resistance. Furthermore, Nb carbides fine precipitation in the grains contributes to the improvement of creep strength. To ensure the excellent naphthenic acid corrosion resistance, Nb content is 0.20% or more.
[0047]
　However, if the content of Nb is excessive, carbides are supposed to excessive deposition in the grain, the deformation of the grains is prevented, further stress concentration in the grain boundary are embrittled by segregation of impurity elements the lead. In particular, when the Nb content exceeds 0.5%, adverse effect is increased as described above. Therefore, in order to ensure high corrosion resistance, Nb content is from 0.20 to 0.50%. Nb content is preferably 0.25% or more is preferably not more than 0.45%.
[0048]
　Al: 0.005 ~
　0.040% Al is an element added as a deoxidizing agent, it is necessary to contain 0.005% or more. However, when content exceeds 0.040% promotes precipitation of intermetallic compounds, toughness and resistance to polythionic acid SCC resistance is deteriorated while being used for a long time. Therefore, Al content is from 0.005 to 0.040%. Al content is preferably at 0.008% or more is preferably not more than 0.035%.
[0049]
　Sn:
　0 ~ 0.080%
　Zn: 0 ~ 0.0060% Pb: 0 ~
　0.030% Sn, Zn and Pb are usually treated as an impurity element as an element adversely affecting the steel quality, these element is high affinity elements with S, because an element effective in improving the resistance to naphthenic acid corrosion resistance, may be contained as needed. However, if excessively contained these elements promote intergranular corrosion, and to provide a reduction in grain boundary strength, thereby rather deteriorate the resistance to naphthenic acid corrosion.
[0050]
　Therefore, Sn, Zn content and Pb, respectively 0.080% or less, and 0.0060% or less and 0.030% or. Sn content is preferably at most 0.050%, Zn content is preferably at most 0.0055%, Pb content is preferably not more than 0.025%. In the case it is desired to obtain the above effect, it is preferable Sn content is 0.002% or more, preferably Zn content is 0.0002% or more, Pb content is less than 0.0005% the preferred there to have.
[0051]
　Incidentally, Sn, taking into account the affinity degree between the respective S Zn and Pb, in order to improve the resistance to naphthenic acid corrosion resistance without adversely affecting the steel quality is defined further below (iv) Formula that it is preferred that the L value satisfies the (v) below formula. = 7.6Sn
　L 0.18 + 9.5Pb 0.18 + 12.8Zn 0.2　　　· · · 　(iv) 1.8 ≦ L ≦ 13.9 · · · (v) 　where the in (iv) Formula element symbol represents the content of each element contained in the steel (mass%).

[0052]
　The proportion of solid solution amount to the total amount of Mo, and in addition to the contents of Ni and Cr, Sn, the content of Zn and Pb is affecting resistance to naphthenic acid corrosion resistance of the steel is as above . The component design in consideration of these balance is desired. Therefore, in relation to the contents of these elements, R is defined by the following (vi) expression 2 is preferably the value satisfies the following (vii) expression.
　R 2 = R 1 + (L-5.1) / 3 · · ·
　(vi) R 2 ≧ 12.0 · · · (vii)
　Here, R in (vi) expression 1 in the above (ii) Formula a defined values, L is a value defined by (iv) above equation.
[0053]
　(B) metal structure
　grain size number: 7.0 or higher
　is not provided any special restriction on austenitic stainless steel metal structure according to the present invention. However, when the crystal grains are coarse, since the HAZ crack susceptibility when welding is increased, the average crystal grain size number of the steel interior defined by ASTM E112 is preferably 7.0 or more. Although not provided particular limitation on the upper limit of the grain size number, to lower the grain is too the creep strength fine is, grain size number inside the steel is preferably 9.5 or less.
[0054]
　(C) the production method
　is not particularly limited production conditions austenitic stainless steel according to the present invention, for example, can be produced by using a method described below.
[0055]
　After melting the steel having the chemical composition described above in a furnace to produce an ingot from the molten steel. And billet forged immediately after heating the ingot. At this time, since the segregation of Mo occurs, subjected to a heat treatment in order to eliminate the segregation by diffusing Mo. The heating temperature at this time is preferably in the range of 1370 ° C. or less exceed 1260 ° C.. The heating temperature is 1260 ° C. or less, it may not be able to sufficiently solid solution Mo are segregated, while when it exceeds 1370 ° C., is because the subsequent working becomes difficult occurs grain boundary melting .
[0056]
　The heating time is preferably set to 8h or more. Is less than the heating time is 8h, there is a possibility that the Mo segregation remains even if heated at a temperature exceeding 1260 ° C.. The upper limit of the heating time is not critical and for too long economy deteriorates, it is preferable to 20h or less. By performing the above heat treatment, it is possible to satisfy the above formula (i) increasing the solid solution ratio of Mo.
[0057]
　Against the billet which has been subjected to the above heat treatment, subjected to hot working. After the heat treatment, it may be directly subjected to hot working, but if the Ni content is low, and remaining part of δ- ferrite, since the hot workability is remarkably lowered, it is preferable to once cool. There is no particular restriction on the cooling rate at this time, it is preferable to cool from the viewpoint of economy. Note that the Mo was once dissolved not be segregated again even gradually cooled in the cooling step.
[0058]
　During the processing between the heat it should be carried out under the following conditions. For example, it can be applied after holding 2 ~ 10h in the temperature range of 1100 ~ 1250 ° C., the hot working such finishing temperature is in the range of 960 ~ 1150 ° C.. Not only the material ductility decreases when the hot working temperature is below 960 ° C., Mo solid solution amount insufficient, can not be obtained resistance naphthenic acid corrosion. After hot working, it may be subjected to cold working for the purpose of such improvement in dimensional accuracy. Further, it is possible to optionally before cold working is subjected to a softening heat treatment. To the grain size number inside the steel and 7.0 or more, for example, a percent reduction of cross section is preferably subjected to cold working under conditions such that 30% or more.
[0059]
　After performing hot working or further cold worked to remove distortion introduced by the process, for the purpose of homogenizing the steel quality of the thickness direction, it performs finishing heat treatment. For grain size number inside the steel to obtain a fine grain metal structure such that 7.0 or more, for example, preferably be heated or 3min in the temperature range of 1000 ~ 1100 ° C.. After finishing the heat treatment is preferably quenched by a method such as water cooling.
[0060]
　For the above austenitic stainless steel produced by the method, by performing various processing, steel, steel pipes, steel plates, steel bars, wire rods, forged steel, it is possible to manufacture the valve, joint and welding material or the like. Since Mo was dissolved once when performing the processing described above never segregated again, impact resistance to naphthenic acid corrosion is considered very small.
[0061]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example
[0062]
　The steel having the chemical components shown in Table 1, were melted using a vacuum induction melting furnace (VIM), to prepare an ingot from the molten steel. Then, test No. 1,3,5-10,13,14,16 For ~ 18 and 20 ~ 24 26 ~ 34 and 37 ~ 39, the width and forged immediately after heating the ingot to 1200 ° C. 100 mm, thickness 50mm It was of the billet. Among the samples of the test No. 1,3,5 The ~ 10,13,14,16 ~ 18,20,31,34,37 to 39, for suppressing the segregation of Mo, as shown in Table 2, 8h at temperatures above 1260 ° C. It was heated above. Test No. And 8h heated at 1260 ° C. for 32, Test No. And 7h heated at 1265 ° C. for 33. In particular heat treatment for the other samples were not subjected.
[0063]
　Thereafter, hot rolling and width 100mm finishing temperature of the billet 900 ~ 1150 ° C., it was steel plate having a thickness of 22 mm. 1080 ± 20 ° C. For these steel sheets, water-cooled immediately after having been subjected to softening heat treatment under the condition of 20 ~ 30min, then cold rolled to produce a width 100 mm, a thickness of 15.4mm steel. The steel plate 1080 ± 20 ° C., and cooled with water immediately after the finish heat treatment under the conditions of 3 ~ 10min, and austenitic stainless steel.
[0064]
　In addition, test No. For 2,4,11,12,15,19,25,27,35 and 36, forged immediately after heating the ingot to 1200 ° C.. Then, the test No. A billet of diameter 320mm for 2,11,19,27 and 36, Test No. For 4,12,15,25 and 35 were the billet diameter 287Mm. Among the samples of the test No. For 2,4,11,12,15,19 and 35, in order to suppress the segregation of Mo, and heated above 8h at a temperature exceeding 1260 ° C. As shown in Table 2. Test No. For 36 and 7h heated at 1350 ° C.. In particular heat treatment for the other samples were not subjected.
[0065]
　Then, the billet test No. For 2,11,19,27 and 36 formed into a hollow billet of diameter 314 mm, internal diameter 47 mm, Test No. For 4,12,15,25 and 35 were molded into a hollow billet of diameter 281 mm, internal diameter 47 mm.
[0066]
　The molded hollow billet extruded at 1250 ~ 1300 ° C., Test No. A diameter 219.5Mm, steel wall thickness 18.3mm for 2,11,19,27 and 36, Test No. For 4,12,15,25 and 35 were diameter 168.7Mm, steel pipe wall thickness 7.0 mm. Then, it was water-cooled immediately after extrusion of the. After the water-cooled, steel pipe in 1000 ~ 1100 ℃, was finished heat treatment under the conditions of 3 ~ 10min. Again it cooled with water immediately after finishing the heat treatment, and the austenitic stainless steel pipe.
[0067]
[Table 1]

[0068]
　For each of the above samples, 20 mA / cm at 10% acetylacetone-1% tetramethylammonium chloride / methanol 2 samples were approximately 0.4g electrolysis at a current of. Then, after the electrolytic solution of the sample was filtered through a filter of 0.2 [mu] m, the residue was acidolysis with a mixed acid of sulfuric acid and phosphoric acid, nitric perchloric acid. Then, a residual amount of Mo by ICP emission spectrophotometer, the Mo content in the molten steel (ladle component value) was determined amount of solid solution Mo by subtracting the residual amount of the Mo. The proportion of the solid solution amount to the total amount of Mo (Mo SS were calculated / Mo).
[0069]
　Furthermore, the relationship between the chemical composition, R is defined by the following (ii) Formula 1 value, R is defined by the L value and the following (vi) expression defined by the following (iv) Formula 2 were calculated values.
　R 1 = 0.25Ni + 0.2Cr + (Mo × (Mo SS / Mo)) 1.45　　　· · ·
　(ii) L = 7.6Sn 0.18 + 9.5Pb 0.18 + 12.8Zn 0.2　　　· · ·
　(iv) R 2 = R 1 + (L-5.1) / 3 · · · (vi)
[0070]
　Next, for each sample of the above was taken test pieces for microstructure observation from within the steel, after polishing a longitudinal section with emery paper and buff, was light microscopy corroded with mixed acid. Grain size number of the observation plane was determined according to the determination method by the comparison method with the grain size standard diagram plate I as defined in ASTM E112. Incidentally, the test pieces for the above tissues observed on that randomly 10 samples taken from the total thickness of the steel, for 10 fields perform optical microscopy, by averaging the grain size number obtained for each, the average It was calculated grain size number.
[0071]
　It also conducts naphthenic acid corrosion test shown below was calculated corrosion rate (mm / y). First, a portion from each sample was collected, the surface was polished with emery paper No. 600 (# 600), to produce a width of 10 mm, a thickness of 3 mm, the corrosion test piece length 30 mm.
[0072]
　Using an autoclave, in a nitrogen (N) atmosphere, an 135Pa, 350 ℃ corrosion test piece in poor quality crude oil were immersed for 48 hours. Poor quality crude oil, and corresponds to the total acid number 6 as defined in ASTM D664-11a. After a lapse of 48 hours, it was taken out corroded specimens. Incidentally, corrosion test because it is consumed acid in inferior quality crude oil TAN value decreases with progresses, corrosion test pieces utilized completely new ones inferior quality crude oil drain outlet and water inlet of the autoclave after soaked 24h replace the, after the immersion test of total 48h, it was removed corroded specimens from the autoclave.
[0073]
　The corrosion test piece after removal from the autoclave, tightly soot was deposited. Therefore, after dropping the 5s subjected robust soot blasting with alumina, 100 ° C. The remaining soot in ammonium citrate solution, dropped by pickling under conditions of 60min. This was followed 3min ultrasonic cleaning with acetone. Then, the weight of the corrosion test piece before the test and the above-described and the weight of the corrosion test piece after ultrasonic cleaning was measured, and the difference was calculated as the corrosion weight loss. Then, to determine the corrosion rate from the surface area and specific gravity, as well as test time of corrosion test piece.
[0074]
　These results are also shown in Table 2. In the present invention, the corrosion rate was to evaluate the superior cases or less 1.50 mm / y in resistance to naphthenic acid corrosion.
[0075]
[Table 2]

[0076]
　Referring to Table 1 and 2, the chemical composition and Mo are defined in the present invention SS study met /Mo≧0.98 No. In 1-20, the corrosion rate is less 1.50 mm / y, it was possible to obtain a desired resistance naphthenic acid corrosion.
[0077]
　Among them, R 1 value, L value and R 2 or one value does not satisfy the preferred range defined in the present invention Test No. The 14 the corrosion rate to 20 are in a greater tendency was confirmed. In particular, R 1 value, L value and R 2 do not satisfy any preferable range of values Test No. Corrosion rate of 14 the most corrosion rate is high in the present invention at 1.50 mm / y was confirmed.
[0078]
　R 1 Since the value is an expression composed of large elements influence the resistance naphthenic acid corrosion such as Cr and Mo, R 1 considered if value does not satisfy the preferred ranges, there is a tendency that the corrosion rate increases It is. Further, L values in order to promote rather intergranular corrosion excess component exceeds the upper limit, it is considered that was increased corrosion rate.
[0079]
　On the other hand, it does not satisfy the requirements of the present invention Test No. In 21-39, the corrosion rate exceeds 1.50 mm / y, resulted in anti naphthenic acid corrosion is poor. In particular, if the Mo content is less than the lower limit defined in the present invention, or, if the heating temperature of the diffusion of Mo is low, or when the heating time is short, Test No. Corrosion rate as 21 to 33 and 36 becomes 1.70 mm / y or more, resulted in the corrosion resistance is degraded. This, Mo is considered to be because in particular have a significant influence on the withstand naphthenic acid corrosion.
Industrial Applicability
[0080]
　According to the present invention without deteriorating the steel quality, it is possible to obtain a with compatibility and excellent naphthenic acid corrosion resistance and economy austenitic stainless steels. Austenitic stainless steel according to the present invention, for example, petroleum refining plant piping, furnaces, distillation columns are suitable for use column, among others components, a pump, as austenitic stainless steel such as a heat exchanger. For example, austenitic stainless steel is steel, steel sheet, steel bars, wire rods, forged steel, valves, fittings, and welding material or the like.

claims
[Requested item 1]
　Chemical composition, in
　mass%, C: 0.015% or
　less, Si: 1.00% or
　less, Mn: 2.00% or
　less, P: 0.05% or
　less, S: 0.030% or
　less, Cr: less 16.0% or more%
　22.0,
　Ni: 11.0 ~ 16.0%, Mo: 2.5
　~ 5.0%, N: less than 0.07% or more% 0.15,
　Nb: 0.
　~
　0.50% 20, Al:
　0.005 ~ 0.040%, Sn: 0 ~ 0.080%,
　Zn: 0 ~ 0.0060%, Pb: 0 ~ 0.030%,
　the balance: Fe and impurities , and the
　satisfy the following formula (i), austenitic stainless steel.
　Mo SS /Mo≧0.98 · · · (i)
　where, Mo in the formula represents the content of Mo contained in the steel (mass%), Mo SS is a solid solution in the steel Mo It represents the content (mass%).
[Requested item 2]
　The chemical composition,
　further R defined by the following (ii) Formula 1 value satisfies (iii) below formula, austenitic stainless steel according to claim 1.
　R 1 = 0.25Ni + 0.2Cr + (Mo × (Mo SS / Mo)) 1.45　　　· · ·
　(ii) R 1 ≧ 10.2 · · · (iii)
　where, (ii) each element symbol in the formula represents the content of each element contained in the steel (mass%), Mo SS represents Mo content in solid solution in the steel (mass%).
[Requested item 3]
　The chemical composition, by
　mass%,
　Sn: 0.002 ~ 0.080%, Zn: 0.0002 ~ 0.0060%, and
　Pb: 0.0005 ~ 0.030%,
　1 or more selected from It contains,
　further below (iv) L value defined by equation satisfies (v) below formula, austenitic stainless steel according to claim 1. = 7.6Sn
　L 0.18 + 9.5Pb 0.18 + 12.8Zn 0.2　　　· · · 　(iv) 1.8 ≦ L ≦ 13.9 · · · (v) 　where the in (iv) Formula element symbol represents the content of each element contained in the steel (mass%).

[Requested item 4]
　The chemical composition, by
　mass%,
　Sn: 0.002 ~ 0.080%, Zn: 0.0002 ~ 0.0060%, and
　Pb: 0.0005 ~ 0.030%,
　1 or more selected from It contains,
　further below (iv) L value defined by equation satisfies (v) below formula, austenitic stainless steel according to claim 2. = 7.6Sn
　L 0.18 + 9.5Pb 0.18 + 12.8Zn 0.2　　　· · · 　(iv) 1.8 ≦ L ≦ 13.9 · · · (v) 　where the in (iv) Formula element symbol represents the content of each element contained in the steel (mass%).

[Requested item 5]
　The chemical composition,
　further R defined by the following (vi) expression 2 value satisfies the following (vii) expression, austenitic stainless steel according to claim 4.
　R 2 = R 1 + (L-5.1) / 3 · · ·
　(vi) R 2 ≧ 12.0 · · · (vii)
　Here, R in (vi) expression 1 in the above (ii) Formula a defined values, L is a value defined by (iv) above equation.
[Requested item 6]
　Grain size number inside the steel has a metal structure is 7.0 or more, austenitic stainless steel according to any one of claims 1 to 5.
[Requested item 7]
　Austenitic stainless steel with austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 8]
　Austenitic stainless steel tube with austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 9]
　Austenitic stainless steel with austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 10]
　Austenitic Sutenresuboko with austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 11]
　Austenitic stainless wire with austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 12]
　Austenitic stainless forged steel with austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 13]
　Austenitic stainless valve using the austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 14]
　Austenitic stainless joints using an austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 15]
　Austenitic stainless welding material with austenitic stainless steel according to any one of claims 1 to 6.
[Requested item 16]
　The steel having the chemical composition according to any one of claims 1 to 5, a heat treatment step of heating 1260 ° C. Beyond 1370 ° C. 8h above at a temperature range,
　range finishing temperature of 960 ~ 1150 ° C. and the hot working step of performing hot working so,
　and a finishing heat treatment step of heating or 3min in the temperature range of 1000 ~ 1100 ° C., a manufacturing method of austenitic stainless steels.
[Requested item 17]
　Further comprising, a manufacturing method of austenitic stainless steel according to claim 16 cold working step of reduction of area is subjected to cold working under conditions such that 30% or more.