Technical field
[0001]
　The present invention relates to austenitic stainless steels, to austenitic stainless steel particularly excellent in acid resistance.
[0002]
　The so-called "fossil fuels" such as oil and coal are used as the boiler fuel for thermal power generation or industrial is contained sulfur (S) is. Therefore, fossil fuel sulfur oxides (SO Exhaust gas when burned x ) is generated. When the temperature of the exhaust gas is lowered, SO x becomes sulfuric acid reacts with the moisture in the gas, condensation at a low temperature of the component surface in below the dew point temperature, thereby resulting sulfuric acid dew-point corrosion.
[0003]
　Similarly, in the flue gas desulfurization equipment used in various industries, SO x when the gas containing flow, sulfuric acid dew point corrosion occurs when the temperature decreases. Hereinafter in this specification, SO for simplicity x a gas containing explained wrote the exhaust gas.
[0004]
　Since the above-mentioned phenomenon occurs in a heat exchanger for use in an exhaust gas system, it retained exhaust gas temperature such that does not bear dew sulfate member surface to 0.99 ° C. or more higher temperatures.
[0005]
　However, from the viewpoint of increasing and effective use of energy in recent years demand for energy in order to as effectively as possible collecting the thermal energy, for example, the exhaust gas temperature from the heat exchanger there is a movement to reduce to below the dew point of sulfuric acid, to sulfuric acid material having resistance Te has come to be demanded.
[0006]
　If it does not hold the exhaust gas temperature above 0.99 ° C., from the exhaust gas of a typical composition in a temperature range of about 140 ° C., a high concentration of sulfuric acid of approximately 80% condensates member surface. In such an environment, so-called "low-alloy steel" has been used as various members steel. This means that for high temperature and high concentration of sulfuric acid as described above, towards the low alloy steel is due to the high corrosion resistance than the general-purpose stainless steel.
[0007]
　On the other hand, as described in Non-Patent Document 1, the amount of sulfuric acid dew condensation region lowered 20 ~ 60 ° C. temperature than the dew point of sulfuric acid is the most, increased corrosion by sulfuric acid. Therefore, if you do not hold the exhaust gas temperature above 0.99 ° C. is generally becomes the area that requires the most corrosion resistance at a temperature of 100 ° C. vicinity, the concentration of sulfuric acid is about 70% here. However, general-purpose stainless steel in this region, of course, not available amount of corrosion is greater in low alloy steel.
[0008]
　Previously, for the member in a sulfuric acid environment, it has been proposed to may be used certain corrosion-resistant material, for example, Patent Document 1, hot workability excellent sulfidation acid dew-point corrosion of stainless steel There has been disclosed.
[0009]
　Further, Patent Document 2, austenitic stainless steels have been disclosed which is excellent in workability and having excellent resistance to sulfuric acid corrosion.
CITATION
Patent Document
[0010]
Patent Document 1: Laid-Open Publication No. 4-346638
Patent Document 2: Patent No. 3294282
Non-patent literature
[0011]
Non-Patent Document 1: Hiroo Nagano, "sulfuric acid dew point corrosion", anti-corrosion technology, 1977, Vol. 26, No. 12, p. 731-740
Summary of the Invention
Problems that the Invention is to Solve
[0012]
　Stainless steel described in Patent Document 1 is to 0.05 wt% or more N (nitrogen) and an attempt is made to secure the stabilization and the corrosion resistance of the austenitic structure. However, when it is contained the N 0.05 wt% or more, Cu, sulfuric acid corrosion resistance of Cr and austenitic stainless steel and Mo added in combination is reduced rather. Furthermore, when the N content is more than 0.05 wt%, when gradually increasing the Cu content in order to increase the sulfuric acid corrosion resistance, significantly decreases the hot workability in the temperature range below 1000 ° C. there is a problem that becomes.
[0013]
　Also, austenitic stainless steel described in Patent Document 2 is one having a processability and excellent resistance to sulfuric acid corrosion resistance. However, for sulfuric acid corrosion resistance, there remains room for further improvement.
[0014]
　The present invention solves the above problems, in environments where high concentrations of sulfuric acid condenses, and an object thereof is to provide austenitic stainless steels having excellent acid resistance.
[0015]
　In the following description, the term "high concentration of sulfuric acid environment condenses" shall mean an environment that condensation 40-70% concentration at a temperature of 50 ~ 100 ° C. sulfate.
Means for Solving the Problems
[0016]
　The present invention has been made to solve the above problems, it is summarized as austenitic stainless steels below.
[0017]
　(1) and the base material, wherein a austenitic stainless steel and a coating formed on at least a portion of a surface base material has,
　the chemical composition of the base material, by mass%,
　C: 0. 0.05% or
　less, Si: 1.0% or
　less, Mn: 2.0% or
　less, P: 0.040% or
　less, S: 0.010% or
　less, O: 0.020% or
　less, N: 0.050%
　less, Ni: 12.0
　~ 27.0%, Cr: less than 15.0% or more%
　20.0, Cu: 8.0% greater than 3.5% or
　less, Mo: more than 2.0% 5.0% or
　less, Co: 0.05% or
　less, Sn: 0.05% or
　less,
　V: 0 ~ 0.5%,
　Nb: 0 ~ 1.0%, Ti: 0 ~
　0.5%, W :
　0
　~
　5.0%, Zr: 0 ~ 1.0%, Al: 0 ~
　0.5%, Ca: 0 ~ 0.01%, B: 0
　~ 0.01%, REM: 0 ~ 0. 01 %,
　The balance is Fe and impurities,
　the chemical composition satisfies the following formula (i) at the maximum Cr depth Cr concentration in the film is maximum,
　austenitic stainless steel.
　(Cr + Ni + Cu + Mo ) /Fe≧1.0 ··· (i)
　wherein each element symbol in the above formula represents the content of each element (at%).
[0018]
　(2) the chemical composition of the base metal, by
　mass%,
　V: 0.01 ~
　0.5%, Nb: 0.02 ~ 1.0%, Ti: 0.01 ~
　0.5%, W:
　~
　5.0% 0.1,
　Zr: 0.02 ~ 1.0%, Al: 0.01 ~ 0.5%, Ca: 0.0005
　~ 0.01%, B: 0.0005 ~ 0. 0.1%,
　and, REM: 0.0005 ~ 0.01%
　containing at least one member selected from,
　austenitic stainless steel according to the above (1).
[0019]
　(3) Minimum Cr depth Cr concentration in the film is minimized, the maximum Cr present in the base material side from the depth,
　the chemical composition in the maximum Cr depth satisfies the following formula (ii), and , the chemical composition satisfies the following formula (iii) in the minimum Cr depth,
　the (1) or austenitic stainless steel according to (2).
　Cr / (Ni + Cu + Mo) ≧ 1.0 · · ·
　(ii) Cr / (Ni + Cu + Mo) <1.0 · · · (iii)
　where each element symbol in the above formula, the content of each element (at%) a representative.
Effect of the invention
[0020]
　According to the present invention, in an environment where a high concentration of sulfuric acid condenses, resulting austenitic stainless steel having excellent acid resistance.
DESCRIPTION OF THE INVENTION
[0021]
　The present inventors, based austenitic stainless steel described in Patent Document 2, a result of intensive studies on how to further improve the sulfuric acid corrosion resistance, the following findings were obtained.
[0022]
　In improving the sulfuric acid corrosion resistance is in contact with a high concentration of sulfuric acid, the composition of the film formed on the base material surface is important. Cr in a film, Ni, by relatively increasing the total content of Cu and Mo with respect to Fe, it is possible to greatly improve the acid resistance.
[0023]
　Further, after forming an oxide film mainly composed of Fe on the surface by heat treatment under predetermined conditions with respect to steel, by virtue of preferential dissolution of Fe component is subjected to acid treatment, Cr in the coating found that it is possible to enrich Ni, Cu and Mo.
[0024]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0025]
　1. Configuration
　austenitic stainless steel according to the present invention comprises a base material and a film formed on at least a portion of the surface of the base material has. For each of the base material and coating, described in detail below.
[0026]
　2. For the base material
　for the chemical composition of the base material will be described in detail. Reasons for limiting each element is as follows. Incidentally, "%" for the content in the following description means "mass%".
[0027]
　C: 0.05% or less
　C is an element having an effect of improving the strength. However, in combination with Cr to form a Cr carbide in grain boundaries, thereby reducing the intergranular corrosion resistance. Therefore, C content is 0.05% or less. In addition, when it is necessary to increase the strength preferably contained more than 0.03%. On the other hand, if the securing of corrosion resistance has priority, C content may lower is preferably set to 0.03% or less. The lower limit is not necessary to particularly provide, in order to obtain the above-mentioned effects, C content is preferably 0.01% or more.
[0028]
　Si: 1.0% or less
　Si is an element having a deoxidizing action. However, to facilitate a reduction in hot workability when its content exceeds 1.0%, I increase coupled with the Cu content, the processing of the product on an industrial scale is extremely difficult. Therefore, Si content is 1.0% or less. Si content is preferably 0.6% or less. Although Si is lower limit is not particularly provided for there is no need to always be contained, in order to obtain the above-mentioned effects, Si content is preferably 0.05% or more. Furthermore, when very low Al content for the purpose of improving the hot workability, it is preferable to sufficiently perform the deoxidation by 0.1% or more Si.
[0029]
　Mn: 2.0% or less
　Mn, as well as improving the hot workability by fixing the S, has the effect of stabilizing the austenitic phase. However, even if Mn is contained in an amount exceeding 2.0%, the effect is saturated, which only costly. Therefore, the Mn content is 2.0% or less. Mn content is preferably 1.5% or less. Although Mn is lower limit is not particularly provided for there is no need to always be contained, in order to obtain the above-mentioned effects, Mn content is preferably 0.1% or more.
[0030]
　P: 0.040% or less
　P is contained in steel in as an impurity, for degrading the hot workability and corrosion resistance, its content is better as low as possible. In particular, when the P content exceeds 0.040%, and remarkable corrosion resistance degradation in environments where high concentrations of sulfuric acid condenses. Accordingly, P content is 0.040% or less.
[0031]
　S: 0.010% or less
　S is included in steel in as an impurity, for degrading the hot workability, the content is better as low as possible. In particular, when the S content exceeds 0.010%, causing the hot workability of significant degradation. Therefore, S content was set to 0.010% or less.
[0032]
　O: 0.020% or less
　O is contained in steel as an impurity, to reduce the hot workability and ductility, the content is better as low as possible. In particular, when the O content exceeds 0.020%, the decrease in hot workability and ductility is significant, O content is 0.020% or less.
[0033]
　N: less than 0.050%
　N is conventionally, N is the object of stabilizing the austenite structure, or have been positively added for the purpose of enhancing the resistance to local corrosion such as pitting or crevice corrosion. However, in environments where high concentrations of sulfuric acid condenses, when the content of N is more than 0.050% Cu exceeding 3.5%, 2.0% more than Mo and 15.0% or more 20.0 corrosion resistance of austenitic stainless steel which contains a Cr of less than% decreases rather. Furthermore, even when the upper limit of the content of Cu and Mo, respectively 8.0% and 5.0%, hot workability if the content of N is more than 0.050% deteriorates . In order to impart a corrosion resistance and hot workability in the environment of high concentrations of sulfuric acid condenses in the austenitic stainless steel, N content should be less than 0.050%. Incidentally, N content is preferably as low as possible is preferably not more than 0.045%.
[0034]
　Ni: 12.0 ~ 27.0% Ni
　is which has the effect of stabilizing the austenitic phase, also act to increase the corrosion resistance in environments where high concentrations of sulfuric acid condenses. In order to sufficiently secure such effects, it is necessary to include 12.0% or more of the amount and Ni. However, when the content exceeds 27.0%, the effect is saturated. In addition, since Ni is an expensive element, lack of economic efficiency and cost becomes very high. Therefore, Ni content is 12.0 to 27.0%. In order to highly concentrated sulfuric acid to ensure sufficient corrosion resistance in an environment condenses is preferably contains a Ni amount of more than 15.0%, thereby containing Ni in an amount exceeding 20.0% it is more preferable.
[0035]
　Cr: less 15.0% than 20.0%
　Cr is an element effective for securing corrosion resistance of austenitic stainless steels. In particular, in the austenitic stainless steel to regulate the N on the content of above 15.0% or more of Cr, and preferably it is contained with the amount of Cu and Mo described below 16.0% or more of Cr, a high concentration of it is possible to ensure a good corrosion resistance in an environment where sulfuric acid condenses. However, if excessively contained the Cr, to lower the N content, even when the Cu and Mo austenitic stainless steel added in combination, the corrosion resistance is degraded rather in the in the environment, further processability also decrease the resulting. In particular, Cr content corrosion degradation of austenitic stainless steel is significantly in the environment and a 20.0% or more. Further, by making the Cr content is less than 20.0%, Cu and Mo to improve the hot workability of austenitic stainless steel with combined addition, to facilitate the product processing on an industrial scale possible to become. Therefore, Cr content is less than 20.0% to 15.0%.
[0036]
　Cu: 8.0% or less beyond the Pasento 3.5
　Cu is an essential element to ensure the corrosion resistance in sulfuric acid in the environment. The Cu in excess of 3.5% by the inclusion with Mo and Cr and described below the amount of the aforementioned amounts, good to the environment in which high concentrations of sulfuric acid condenses, austenitic stainless steel in which the N on the content of above it is possible to impart corrosion resistance. Since the larger the content of Cu to be added in combination with Cu and Mo corrosion resistance improvement effect is large, Cu content is preferably an amount exceeding 4.0%. Incidentally, corrosion resistance in the environment by increasing the Cu content is increased but then decreased hot workability, particularly, when the Cu content exceeds 8.0%, even if the N on the content of above produce the hot workability of significant degradation. Therefore, Cu content is 8.0% or less than 3.5%.
[0037]
　Mo: 5.0% or less exceed 2.0%
　Mo is an effective element for ensuring corrosion resistance of austenitic stainless steels. When the amount of Mo exceeding 2.0% is contained together with Cr and Cu in an amount described above, in an environment in which high concentrations of sulfuric acid condenses, good corrosion resistance in austenitic stainless steel in which the N on the content of above grant can do. However, if excessively contained the Mo lowered hot workability, particularly, when the Mo content exceeds 5.0%, even if the N on the content of above produce the hot workability of significant degradation. Therefore, Mo content is 5.0% more than 2.0% or less. In order to highly concentrated sulfuric acid to ensure sufficient corrosion resistance in an environment condenses, it is preferred to include a Mo in an amount exceeding 3.0%.
[0038]
　Co: 0.05% or less
　Co is an element contained in steel as an impurity. Co is an element effective to increase the toughness of the steel, is therefore not necessary to add positively an expensive element. Therefore, Co content is 0.05% or less.
[0039]
　Sn: 0.05% or less
　Sn is contained in steel as an impurity, for degrading the hot workability, the content is better as low as possible. In particular, when the Sn content exceeds 0.05%, causing the hot workability of significant degradation. Accordingly, Sn content is 0.05% or less.
[0040]
　V: 0.5% or less
　V is corrosion by fixing the C, since it has an effect of enhancing inter alia intergranular corrosion resistance, it may be contained as needed. However, when the content exceeds 0.5%, rather it decreases corrosion resistance generates nitrides even when the N on the content of above, further, also result in hot workability occurs. Therefore, V content is 0.5% or less. To obtain the above-mentioned effects, V content is preferably 0.01% or more.
[0041]
　Nb: 0 ~ 1.0% Nb
　is corrosion by fixing the C, since it has an effect of enhancing inter alia intergranular corrosion resistance, may be contained as needed. However, if its content exceeds 1.0%, rather it decreases corrosion resistance generates nitrides even when the N on the content of above, further, also result in hot workability occurs. Therefore, Nb content is 1.0% or less. To obtain the above effect, Nb content is preferably 0.02% or more.
[0042]
　Ti: 0 ~ 0.5% Ti
　is corrosion by fixing C as with Nb, since it has an effect of enhancing inter alia intergranular corrosion resistance, may be contained as needed. However, when the content exceeds 0.5%, rather it decreases corrosion resistance generates nitrides even when the N on the content of above, further, also result in hot workability occurs. Therefore, Ti content is 0.5% or less. To obtain the above-mentioned effects, Ti content is preferably 0.01% or more.
[0043]
　W: 0 ~ 5.0%
　W, since the high concentration of sulfuric acid has an action to enhance corrosion resistance in an environment condenses, it may be contained as necessary. However, when the content exceeds 5.0%, the above effect is saturated, which only costly. Therefore, W content is 5.0% or less. To obtain the above-mentioned effects, W content is preferably 0.1% or more.
[0044]
　Zr: 0 ~ 1.0%
　Zr, since having a function of improving the corrosion resistance in an environment in which high concentrations of sulfuric acid condenses, may be contained as necessary. However, if its content exceeds 1.0%, the above effect is saturated, which only costly. Accordingly, Zr content is 1.0% or less. To obtain the above-mentioned effects, Zr content is preferably 0.02% or more.
[0045]
　Al: 0 ~
　0.5% Al is, since it has a deoxidizing effect, if kept very low Si content, may be contained. However, when the content exceeds 0.5%, hot workability even austenitic stainless steel in which the N on the content of above is reduced. Therefore, Al content is 0.5% or less. The lower limit of the Al content is not particularly limited, and may be in the range of impurities. However, when kept very low Si content is actively contain a least 0.02% addition, it is preferable to sufficiently perform the deoxidation. Even when which contains 0.05% or more Si, in order to realize the best deoxidation effect, the Al content is preferably set to 0.01% or more.
[0046]
　Ca: 0 ~ 0.01%
　Ca, in order to have the effect of suppressing the binding and reduction in hot workability and S, may be contained as needed. However, when the content exceeds 0.01%, cleanliness of the steel is lowered, causing the flaw generated during production in hot. Therefore, Ca content is 0.01% or less. To obtain the above effect, Ca content is preferably 0.0005% or more, more preferably 0.001% or more.
[0047]
　B: 0 ~ 0.01%
　B, since an effect of improving the hot workability, may be contained as needed. However, excessive addition of B promotes the precipitation of Cr-B compound in the grain boundaries, leading to corrosion deterioration. In particular, the content of B is lead to significant corrosion of deterioration exceeds 0.01%. Therefore, B content is 0.01% or less. To obtain the above-mentioned effects, B content is preferably 0.0005% or more, more preferably 0.001% or more.
[0048]
　REM: 0 ~
　0.01% REM (rare earth element), in order to have an effect of improving the hot workability, may be contained as needed. However, when the content exceeds 0.01%, cleanliness of the steel is lowered, causing the flaw generated during production in hot. Therefore, REM content is set to 0.01% or less. To obtain the above effect, REM content is preferably 0.0005% or more.
[0049]
　Here, the REM, Sc, refers to a total of 17 elements of Y and lanthanoid, and the content of REM refers to the total content of these elements.
[0050]
　In the chemical composition of the base material of austenitic stainless steel of the present invention, the balance being Fe and impurities. Here, the "impurities", in manufacturing the steel industrially, ores, raw material scraps, a component mixed by various factors of the manufacturing process, is allowed to the extent that the present invention does not adversely affect means shall.
[0051]
　3. For coating
　, as described above, at least part of the surface of the base material has the coating is formed. Then, by relatively increasing Cr, Ni, the total content of Cu and Mo on Fe in a coating, it is possible to greatly improve the acid resistance.
[0052]
　Specifically, in the coating, there is the maximum Cr depth Cr concentration is maximum, the chemical composition of the maximum Cr depth is required to satisfy the following formula (i). Incidentally, no particular limitation is imposed on the position of the maximum Cr depth, may be present in the outermost layer of the film.
　(Cr + Ni + Cu + Mo ) /Fe≧1.0 ··· (i)
　wherein each element symbol in the above formula represents the content of each element in the steel surface (at%).
[0053]
　Further, the film according to the present invention has a structure which generally Cr relatively thickened surface layer side of the layer and Ni or the like and a layer of relatively thickened preform side. That is, the base material side of the maximum Cr depth above, so that the minimum Cr depth Cr concentration is minimum is present.
[0054]
　Then, the chemical composition at the maximum Cr depth, it is preferred to satisfy the following formula (ii), the chemical composition at the minimum Cr depth, it is preferable to satisfy the following formula (iii).
　Cr / (Ni + Cu + Mo) ≧ 1.0 · · ·
　(ii) Cr / (Ni + Cu + Mo) <1.0 · · · (iii)
　where each element symbol in the above formula, the content of each element (at%) a representative.
[0055]
　Although not provided any special restriction on the thickness of the film, for example, it is preferably in the range of 2 ~ 10 nm. In less than 2nm thickness of the film, there is a fear that sulfuric acid corrosion resistance is not sufficiently obtained. On the other hand, if the thickness of the film exceeds 10 nm, because the separation of the heterogeneous and coating of the coating composition may become likely to occur.
[0056]
　In the present invention, the chemical composition of the coating shall be measured by the depth analysis using X-ray photoelectron spectroscopy (XPS). The above depth analysis, derived as the ratio (at%) occupying the concentration profile of each element O, the components except C and N. Then, by specifying the maximum Cr depth and minimum Cr depth, determine the concentration of each element in the depth, calculated from the equation their values ​​(i) ~ (iii).
[0057]
　The thickness of the coating, and that determined from the concentration profiles of O (oxygen). Specifically, the concentration a position of 1/3 of the maximum concentration of O, determines that the boundary between the coating and the base material, the length from the coating surface to said boundary portion, the thickness of the coating . Measurement of composition and thickness of the coating is carried out in multiple locations, it is desirable to employ the average value.
[0058]
　4. Production process
　is not particularly limited production conditions austenitic stainless steel according to the present invention, for example, with respect to a steel material having a chemical composition described above, by heat treatment and acid treatment under the conditions shown below, to produce be able to.
[0059]
　
　with respect to the steel material, first 1060 temperature range of ~ 1140 ° C. in applying 60 ~ 600s heat treatment for holding. Thus, to form an oxide film mainly composed of Fe in the steel material surface. The heat treatment temperature is formation of Fe oxide film is insufficient at less than 1060 ° C.. On the other hand, when the heat treatment temperature exceeds 1140 ° C. of the base material crystal grains become coarse, the diffusion of Fe is reduced, Fe oxide film becomes uneven, it tends to occur more decapsulation. As a result, in the case of any of the above also, Cr, Ni, is enrichment of Cu and Mo is unlikely to occur.
[0060]
　
　with respect to the steel material, subsequent to the above heat treatment, subjected to acid treatment. In the acid treatment step, by preferential dissolution of Fe component, it is possible to concentrated Cr, Ni, Cu and Mo to the steel surface. The Fe component in order to preferentially dissolve, 30 ~ 50 ° C., 5-8 vol% HNO 3 preferably be 1 ~ 5h immersed in hydrofluoric nitric acid, 5-8 vol% HF.
[0061]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example
[0062]
　Steel having the chemical compositions shown in Table 1 (steel No.1 ~ 11) was melted using a VIM melting furnace 3.5t, hot forging in the usual way, by performing the hot extrusion and cold drawing , an outer diameter of 75mm, the steel pipe material of the wall thickness of 3mm was fabricated. Then, test No. About 1 to 17 and 19 to 28, subjected to a heat treatment and acid treatment under the conditions shown in Table 2, and austenitic stainless steel. In addition, test No. For 18, test No. 3 and was subjected to a heat treatment and acid treatment under the same conditions, and polishing the surface.
[0063]
[Table 1]

[0064]
[Table 2]

[0065]
　Next, the measurement of the chemical composition and thickness of the film formed on the steel surface was performed by depth analysis using XPS. Specifically, the concentration profile of each element O, after derived as a percentage (at%) to total components excluding C and N, was identified maximum Cr depth and minimum Cr depth, each element in the depth to determine the concentration. Then, it was calculated from these values ​​above formula (i) ~ (iii). In the present embodiment, the test No. In the example except 18, there are maximum Cr depth the outermost layer of the film, also, in all instances, the minimum Cr depth was present in the base material side of the maximum Cr depth.
[0066]
　The thickness of the coating was determined from the concentration profiles of O (oxygen). Specifically, the concentration a position of 1/3 of the maximum concentration of O, determines that the boundary between the coating and the base material, the length from the coating surface to said boundary portion, and a thickness of the coating .
[0067]
　Further, since the evaluation of sulfuric acid corrosion resistance was carried out corrosion tests in a sulphate environment. Corrosion test was conducted by temperature immersing the steel in a solution of 70% sulfuric acid concentration at 100 ° C.. Then measure the corrosion loss after immersing for 8 hours was calculated corrosion rate per unit area. In the present invention, the corrosion rate 1.00 g / (m 2 was that when a · h) or less, it is determined that excellent resistance to sulfuric acid corrosion resistance.
[0068]
　The results are shown together in Table 3.
[0069]
[table 3]

[0070]
　As can be seen from Table 3, the test production conditions are inappropriate No. Test of 1, 2 and 14-17, as well as polishing the skin No. In 18, since Cr in the coating, Ni, is enrichment of Cu and Mo does not occur, the corrosion rate is high, has resulted in sulfuric acid corrosion resistance is poor. Similarly, the test Cu content in the base material is out of the provisions of the present invention No. In 28, in addition to not obtained acid resistance by Cu, because Cr in the coating, Ni, is enrichment of Cu and Mo was insufficient, resulted in sulfuric acid corrosion resistance is poor.
[0071]
　For these, satisfying the requirements of the present invention, the test Cr in the coating, Ni, Cu and Mo were concentrated No. In 3 to 13 and 19-27, the corrosion rate of 1.00 g / (m 2 becomes · h) or less, resulted in excellent resistance to sulfuric acid corrosion resistance.
Industrial Applicability
[0072]
　According to the present invention, in an environment where a high concentration of sulfuric acid condenses, resulting austenitic stainless steel having excellent acid resistance. Accordingly, the austenitic stainless steel according to the present invention, the heat exchangers used in thermal power generation or industrial boilers, is used in the flue and chimney, and flue gas desulfurization apparatus for members or sulfuric acid which must be used by various industries that is applicable to various members such as structural members for use in equipment.

The scope of the claims
[Requested item 1]
　And the base material, a austenitic stainless steel and a coating formed on at least a portion of a surface of said base material has,
　the chemical composition of the base material, in mass%,
　C: 0.05% or less ,
　Si: 1.0% or
　less, Mn: 2.0% or
　less, P: 0.040% or
　less, S: 0.010% or
　less, O: 0.020% or
　less, N: less than
　0.050%, Ni : 12.0
　~ 27.0%, Cr: less than 15.0% or more%
　20.0, Cu: 8.0% greater than 3.5% or
　less, Mo: more than 2.0% 5.0 % or
　less, Co: 0.05% or
　less, Sn: 0.05% or
　less,
　V: 0 ~ 0.5%,
　Nb: 0 ~ 1.0%, Ti: 0 ~
　0.5%, W: 0 ~
　%
　5.0,
　Zr: 0 ~ 1.0%, Al:
　0 ~ 0.5%, Ca: 0 ~ 0.01%,
　B: 0 ~ 0.01%, REM: 0 ~ 0.01%,
　the remaining Parts: an Fe and impurities,
　the chemical composition satisfies the following formula (i) at the maximum Cr depth Cr concentration in the film is maximum,
　austenitic stainless steel.
　(Cr + Ni + Cu + Mo ) /Fe≧1.0 ··· (i)
　wherein each element symbol in the above formula represents the content of each element (at%).
[Requested item 2]
　Chemical composition of the base metal, by
　mass%,
　V: 0.01 ~
　0.5%, Nb: 0.02 ~ 1.0%, Ti: 0.01 ~
　0.5%, W: 0.1 　5.0%
~, 　Zr: 0.02 ~ 1.0%, Al: 0.01 ~ 0.5%, 　Ca: 0.0005 ~ 0.01%, B: 0.0005 ~ 0.01%, 　and, REM: 0.0005 ~ 0.01%, 　containing one or more selected from, 　austenitic stainless steel according to claim 1.

[Requested item 3]
　Minimum Cr depth Cr concentration in the film is minimized, the maximum Cr present in the base material side from the depth,
　the chemical composition in the maximum Cr depth satisfies the following formula (ii), and the minimum chemical composition satisfies the following formula (iii) in the Cr depth
　claim 1 or austenitic stainless steel according to claim 2.
　Cr / (Ni + Cu + Mo) ≧ 1.0 · · ·
　(ii) Cr / (Ni + Cu + Mo) <1.0 · · · (iii)
　where each element symbol in the above formula, the content of each element (at%) a representative.