CN109563589B - Austenitic stainless steel - Google Patents

Austenitic stainless steel Download PDF

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CN109563589B
CN109563589B CN201780048557.4A CN201780048557A CN109563589B CN 109563589 B CN109563589 B CN 109563589B CN 201780048557 A CN201780048557 A CN 201780048557A CN 109563589 B CN109563589 B CN 109563589B
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stainless steel
sulfuric acid
austenitic stainless
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CN109563589A (en
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栗原伸之佑
相良雅之
小薄孝裕
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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Abstract

An austenitic stainless steel, comprising: a base material and a coating film formed on at least a part of a surface of the base material, wherein the chemical composition of the base material is, 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 to 27.0%, Cr: 15.0% or more and less than 20.0%, Cu: more than 3.5% and 8.0% or less, Mo: greater than 2.0% and less than 5.0%, 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%, and the balance: fe and impurities, and the chemical composition at the maximum Cr depth of the coating film is calculated by at percent and meets [ (Cr + Ni + Cu + Mo)/Fe is more than or equal to 1.0 ].

Description

Austenitic stainless steel
Technical Field
The present invention relates to austenitic stainless steel, and particularly to austenitic stainless steel having excellent acid resistance.
Sulfur (S) is contained in so-called "fossil fuels" such as petroleum and coal, which are used as boiler fuels for thermal power generation and industry. Therefore, if fossil fuels are burned, Sulfur Oxides (SO) are produced in exhaust gasx). If the temperature of the exhaust gas is lowered, SOxThe sulfuric acid reacts with moisture in the gas to form sulfuric acid, and the sulfuric acid forms dew point corrosion by condensation on the surface of the member at a low temperature equal to or lower than the dew point temperature.
Also, even thoughIn flue gas desulfurization apparatuses used in various industries, a stream of flue gas contains SOxThe sulfuric acid dew point corrosion also occurs when the temperature of the gas (2) is lowered. In the following description, SO will be included for the sake of simplicityxThe gas of (2) will be described as an exhaust gas.
Since the above phenomenon occurs, in a heat exchanger or the like used in an exhaust gas system, the exhaust gas temperature is maintained at a high temperature of 150 ℃ or higher so that the sulfuric acid does not condense on the surface of the member.
However, from the viewpoint of the recent increase in energy demand and effective utilization of energy, there is a trend to reduce the temperature of exhaust gas from a heat exchanger to a temperature equal to or lower than the dew point of sulfuric acid, for example, in order to recover thermal energy as efficiently as possible, and materials having resistance to sulfuric acid are increasingly required.
When the exhaust gas temperature is not maintained at 150 ℃ or higher, about 80% of high-concentration sulfuric acid forms dew on the surface of the member in a temperature range of about 140 ℃ of the exhaust gas having a general composition. Under such circumstances, so-called "low alloy steels" are gradually beginning to be used as steels for various members. This is because the corrosion resistance of the low alloy steel is higher than that of the general-purpose stainless steel with respect to the high-temperature and high-concentration sulfuric acid as described above.
On the other hand, as described in non-patent document 1, in a region at a temperature 20 to 60 ℃ lower than the dew point of sulfuric acid, the amount of sulfuric acid condensed becomes the largest, and therefore, corrosion by sulfuric acid becomes large. Therefore, when the exhaust gas temperature is not maintained at 150 ℃ or higher, the temperature of about 100 ℃ generally becomes the region where the corrosion resistance is most required, and the concentration of sulfuric acid here becomes about 70%. However, in this region, not only general-purpose stainless steel but also low alloy steel has a large amount of corrosion, and thus cannot be used.
Hitherto, it has been proposed to use a specific corrosion-resistant material for a member exposed to sulfuric acid, and for example, patent document 1 discloses a sulfuric acid dew point corrosion resistant stainless steel excellent in hot workability
Further, patent document 2 discloses an austenitic stainless steel having excellent resistance to sulfuric acid corrosion and also having excellent workability.
Documents of the prior art
Patent document
Patent document 1: japanese laid-open patent publication No. 4-346638
Patent document 2: japanese patent No. 3294282
Non-patent document
Non-patent document 1: changebofu, "sulfuric acid dew point corrosion", anticorrosion technique, 1977, volume 26, No. 12, p.731-740
Disclosure of Invention
Problems to be solved by the invention
The stainless steel described in patent document 1 contains 0.05 wt% or more of N (nitrogen) in an attempt to ensure the stabilization of the austenitic structure and the corrosion resistance. However, when N is contained in an amount of 0.05 wt% or more, the corrosion resistance to sulfuric acid of the austenitic stainless steel to which Cu, Cr, and Mo are added in combination is rather lowered. Further, when the N content is 0.05 wt% or more, if the Cu content is increased in order to improve the sulfuric acid corrosion resistance, there is a problem that the hot workability is remarkably lowered in a temperature range of less than 1000 ℃.
In addition, the austenitic stainless steel described in patent document 2 has excellent sulfuric acid corrosion resistance and workability. However, there is still room for further improvement in the corrosion resistance to sulfuric acid.
The present invention has been made to solve the above problems, and an object thereof is to provide austenitic stainless steel having excellent acid resistance in an environment where sulfuric acid of high concentration is condensed.
In the following description, the "environment in which high-concentration sulfuric acid is condensed" refers to an environment in which condensation of 40 to 70% concentration sulfuric acid occurs at a temperature of 50 to 100 ℃.
Means for solving the problems
The present invention is made to solve the above problems, and the gist thereof is austenitic stainless steel described below.
(1) An austenitic stainless steel, comprising: a base material and a coating film formed on at least a part of a surface of the base material,
the chemical composition of the base material is calculated by mass%
C: less than 0.05 percent of,
Si: less than 1.0 percent,
Mn: less than 2.0 percent,
P: less than 0.040%,
S: less than 0.010%,
O: less than 0.020%,
N: less than 0.050%,
Ni：12.0～27.0％、
Cr: more than 15.0 percent and less than 20.0 percent,
Cu: more than 3.5% and not more than 8.0%,
Mo: more than 2.0% and not more than 5.0%,
Co: less than 0.05 percent of,
Sn: less than 0.05 percent of,
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％、
And the balance: fe and impurities in the iron-based alloy, and the impurities,
the chemical composition at the maximum Cr depth at which the Cr concentration of the coating film becomes maximum satisfies the following formula (i).
(Cr+Ni+Cu+Mo)/Fe≥1.0···(i)
Wherein each element symbol in the above formula represents the content (at%) of each element.
(2) The austenitic stainless steel according to the item (1), wherein the chemical composition of the base material contains, in mass%, a chemical component selected from the group consisting of
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 to 0.01%, and
REM：0.0005～0.01％
1 or more of them.
(3) The austenitic stainless steel according to the item (1) or (2), wherein a minimum Cr depth at which the Cr concentration of the coating film is minimum is present on the base material side of the maximum Cr depth,
the chemical composition at the maximum Cr depth satisfies the following formula (ii), and the chemical composition at the minimum Cr depth satisfies the following formula (iii).
Cr/(Ni+Cu+Mo)≥1.0···(ii)
Cr/(Ni+Cu+Mo)<1.0···(iii)
Wherein each element symbol in the above formula represents the content (at%) of each element.
ADVANTAGEOUS EFFECTS OF INVENTION
According to the present invention, an austenitic stainless steel having excellent acid resistance in an environment where sulfuric acid of high concentration is condensed can be obtained.
Detailed Description
The present inventors have made extensive studies on a method for further improving the corrosion resistance to sulfuric acid based on the austenitic stainless steel described in patent document 2, and as a result, have obtained the following findings.
In order to improve the corrosion resistance to sulfuric acid, the composition of a coating film formed on the surface of a base material in contact with a high concentration of sulfuric acid is important. In the coating, the total content of Cr, Ni, Cu, and Mo with respect to Fe is relatively increased, whereby the acid resistance can be greatly improved.
Further, it has been found that Cr, Ni, Cu and Mo can be enriched in a film by subjecting steel to heat treatment under predetermined conditions to form an oxide film mainly containing Fe on the surface, and then subjecting the steel to acid treatment to preferentially melt the Fe component.
The present invention has been made based on the above findings. Hereinafter, each feature of the present invention will be described in detail.
1. Form a
The austenitic stainless steel of the present invention comprises: the coating film is formed on at least a part of the surface of the base material. The base material and the coating film will be described in detail below.
2. About a base material
The chemical composition of the base material will be described in detail. The reasons for limiting the elements are as follows. In the following description, "%" as to the content means "% by mass".
C: less than 0.05%
C is an element having an effect of improving strength. However, Cr bonds with Cr to form Cr carbide in grain boundaries, and the intergranular corrosion resistance is lowered. Therefore, the C content is set to 0.05% or less. When the strength needs to be improved, the content is preferably more than 0.03%. On the other hand, when priority is given to ensuring corrosion resistance, the C content is preferably low, and preferably 0.03% or less. The lower limit is not particularly required, but the C content is preferably 0.01% or more in order to obtain the above effects.
Si: 1.0% or less
Si is an element having a deoxidizing effect. However, if the content exceeds 1.0%, the reduction of hot workability is promoted, and the decrease interacts with the increase of the Cu content, so that the processing into products on an industrial scale becomes extremely difficult. Therefore, the Si content is 1.0% or less. The Si content is preferably 0.6% or less. The lower limit is not particularly limited since Si is not necessarily contained, but the Si content is preferably 0.05% or more in order to obtain the above effects. In addition, when the Al content is extremely reduced for the purpose of improving hot workability, it is preferable that 0.1% or more of Si is contained so that the deoxidation effect is sufficiently performed.
Mn: 2.0% or less
Mn has the function of fixing S to improve hot workability and stabilize the austenite phase. However, even if Mn is contained in an amount exceeding 2.0%, the effect is saturated, and only the cost is increased. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably 1.5% or less. The lower limit is not particularly limited since Mn is not necessarily contained, but in order to obtain the above effects, the Mn content is preferably 0.1% or more.
P: less than 0.040%
P is contained as an impurity in the steel, and deteriorates hot workability and corrosion resistance, and therefore, the content thereof is preferably as low as possible. In particular, if the P content exceeds 0.040%, the deterioration of the corrosion resistance in an environment where high-concentration sulfuric acid condenses becomes significant. Therefore, the P content is set to 0.040% or less.
S: 0.010% or less
S is contained as an impurity in the steel to deteriorate hot workability, and therefore, the content thereof is preferably as low as possible. Particularly, if the S content exceeds 0.010%, the hot workability is remarkably deteriorated. Therefore, the S content is set to 0.010% or less.
O: 0.020% or less
O is contained in the steel as impurities to lower hot workability and ductility, and the content thereof is preferably as low as possible. In particular, if the O content exceeds 0.020%, the hot workability and ductility are remarkably reduced, so that the O content is 0.020% or less.
N: less than 0.050%
N has been added actively for the purpose of stabilizing the austenite structure or for the purpose of improving the resistance to local corrosion such as pitting corrosion and crevice corrosion. However, in an environment where high-concentration sulfuric acid is condensed, when the N content is 0.050% or more, the corrosion resistance of the austenitic stainless steel containing Cu of more than 3.5%, Mo of more than 2.0%, and Cr of 15.0% or more and less than 20.0% is rather lowered. Further, even when the upper limits of the contents of Cu and Mo are set to 8.0% and 5.0%, respectively, the hot workability is lowered if the content of N is 0.050% or more. In order to impart corrosion resistance and hot workability under an environment in which a high concentration of sulfuric acid is condensed to austenitic stainless steel, the content of N is set to less than 0.050%. The lower the N content, the better, preferably 0.045% or less.
Ni：12.0～27.0％
Ni has an effect of stabilizing an austenite phase and also has an effect of improving corrosion resistance in an environment where high-concentration sulfuric acid is condensed. In order to sufficiently ensure such an effect, Ni needs to be contained in an amount of 12.0% or more. However, even if the content exceeds 27.0%, the effect is saturated. Further, since Ni is an expensive element, the cost is extremely high, and the economy is poor. Therefore, the Ni content is set to 12.0 to 27.0%. In order to ensure sufficient corrosion resistance in an environment where high-concentration sulfuric acid is condensed, Ni is preferably contained in an amount exceeding 15.0%, and more preferably contained in an amount exceeding 20.0%.
Cr: more than 15.0 percent and less than 20.0 percent
Cr is an element effective for ensuring the corrosion resistance of austenitic stainless steel. In particular, in austenitic stainless steel in which N is limited to the above-described content, if 15.0% or more of Cr, preferably 16.0% or more of Cr is contained together with Cu and Mo in amounts described later, good corrosion resistance can be ensured in an environment where high-concentration sulfuric acid is condensed. However, if Cr is excessively contained, even in the case of an austenitic stainless steel in which the N content is reduced and Cu and Mo are compositely added, the corrosion resistance in the above-described environment is rather deteriorated, and further, the workability is also lowered. In particular, if the Cr content is 20.0% or more, the corrosion resistance of the austenitic stainless steel in the above-described environment is remarkably deteriorated. Further, by making the Cr content less than 20.0%, the hot workability of the austenitic stainless steel to which Cu and Mo are compositely added can be improved, and the product can be easily processed on an industrial scale. Therefore, the Cr content is set to 15.0% or more and less than 20.0%.
Cu: more than 3.5% and less than 8.0%
Cu is an element necessary for ensuring corrosion resistance in a sulfuric acid environment. By containing more than 3.5% of Cu together with the above-described amount of Cr and the below-described amount of Mo, it is possible to impart good corrosion resistance to austenitic stainless steel in which N is contained in the above-described amount under an environment where high-concentration sulfuric acid is condensed. The Cu content is preferably set to an amount exceeding 4.0% because the corrosion resistance improving effect increases as the Cu content increases when Cu and Mo are added in combination. Although the corrosion resistance in the above-described environment is improved by increasing the Cu content, the hot workability is deteriorated, and particularly if the Cu content exceeds 8.0%, the hot workability is remarkably deteriorated even if the N content is set to the above-described content. Therefore, the Cu content is set to be more than 3.5% and 8.0% or less.
Mo: more than 2.0% and less than 5.0%
Mo is an element effective for ensuring the corrosion resistance of austenitic stainless steel. When Mo is contained in an amount exceeding 2.0% together with Cr and Cu in the above amounts, good corrosion resistance can be imparted to austenitic stainless steel in which N is contained in the above amount under an environment where sulfuric acid of high concentration is condensed. However, if Mo is excessively contained, hot workability is deteriorated, and particularly if the Mo content exceeds 5.0%, even if N is contained in the above amount, hot workability is remarkably deteriorated. Therefore, the Mo content is set to be more than 2.0% and 5.0% or less. In order to ensure sufficient corrosion resistance in an environment where high-concentration sulfuric acid is condensed, Mo is preferably contained in an amount exceeding 3.0%.
Co: less than 0.05%
Co is an element contained in the steel as an impurity. Co is an element effective for improving the toughness of steel, but it is an expensive element, and therefore, it is not necessary to add it actively. Therefore, the Co content is set to 0.05% or less.
Sn: less than 0.05%
Sn is contained as an impurity in steel, and the hot workability is deteriorated, so the content thereof is preferably as low as possible. Particularly, if the Sn content exceeds 0.05%, the hot workability is remarkably deteriorated. Therefore, the Sn content is 0.05% or less.
V: less than 0.5%
V has an effect of fixing C to improve corrosion resistance, particularly intergranular corrosion resistance, and therefore can be contained as needed. However, if the content exceeds 0.5%, even if N is contained in the above-mentioned amount, nitrides are generated, whereby the corrosion resistance is rather lowered, and further, the hot workability is deteriorated. Therefore, the V content is set to 0.5% or less. In order to obtain the above effects, the V content is preferably 0.01% or more.
Nb：0～1.0％
Nb has an effect of fixing C to improve corrosion resistance, particularly grain boundary corrosion resistance, and therefore can be contained as needed. However, if the content exceeds 1.0%, even if N is contained in the above-mentioned amount, nitrides are generated, whereby the corrosion resistance is rather lowered, and further, the hot workability is deteriorated. Therefore, the Nb content is 1.0% or less. In order to obtain the above effects, the Nb content is preferably 0.02% or more.
Ti：0～0.5％
Ti has the effect of fixing C to improve corrosion resistance, particularly grain boundary corrosion resistance, similarly to Nb, and therefore may be contained as necessary. However, if the content exceeds 0.5%, even if N is contained in the above-mentioned amount, nitrides are generated, whereby the corrosion resistance is rather lowered, and further, the hot workability is deteriorated. Therefore, the Ti content is set to 0.5% or less. In order to obtain the above effects, the Ti content is preferably 0.01% or more.
W：0～5.0％
W has an effect of improving corrosion resistance in an environment where high-concentration sulfuric acid is condensed, and therefore, may be contained as needed. However, if the content exceeds 5.0%, the above effect is saturated, and only the cost increases. Therefore, the W content is 5.0% or less. In order to obtain the above effects, the W content is preferably 0.1% or more.
Zr：0～1.0％
Zr has an effect of improving corrosion resistance in an environment where high-concentration sulfuric acid is condensed, and therefore, it may be contained as necessary. However, if the content exceeds 1.0%, the above effect is saturated, and only the cost increases. Therefore, the Zr content is 1.0% or less. In order to obtain the above effects, the Zr content is preferably 0.02% or more.
Al：0～0.5％
Since Al has a deoxidizing effect, it may be contained in a case where the Si content is extremely low. However, if the content exceeds 0.5%, the hot workability of the austenitic stainless steel is lowered even if the content of N is set to the above-mentioned content. Therefore, the Al content is set to 0.5% or less. The lower limit of the Al content is not particularly limited, and may be in the range of impurities. In the case where the Si content is suppressed to be extremely low, it is preferable to positively add the Si content to 0.02% or more so that the deoxidation effect is sufficiently performed. Even when 0.05% or more of Si is contained, the Al content is preferably 0.01% or more in order to sufficiently exhibit the deoxidation effect.
Ca：0～0.01％
Ca and S bind to each other to have an effect of suppressing a decrease in hot workability, and therefore, may be contained as needed. However, if the content exceeds 0.01%, the cleanliness of the steel is reduced, which causes defects in the production by heat. Therefore, the Ca content is 0.01% or less. In order to obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more.
B：0～0.01％
B has an effect of improving hot workability, and therefore, may be contained as necessary. However, excessive addition of B promotes precipitation of Cr — B compounds in grain boundaries, resulting in deterioration of corrosion resistance. Particularly, if the content of B exceeds 0.01%, significant deterioration in corrosion resistance is caused. Therefore, the B content is set to 0.01% or less. In order to obtain the above effects, the B content is preferably 0.0005% or more, more preferably 0.001% or more.
REM：0～0.01％
REM (rare earth element) has an effect of improving hot workability, and therefore, may be contained as necessary. However, if the content exceeds 0.01%, the cleanliness of the steel is reduced, which causes defects in the production by heat. Therefore, the REM content is set to 0.01% or less. In order to obtain the above effects, the REM content is preferably 0.0005% or more.
Here, REM means 17 elements in total of Sc, Y and lanthanoid, and the content of REM means the total content of these elements.
In the chemical composition of the parent material of the austenitic stainless steel of the present invention, the balance is Fe and impurities. Here, "impurities" means: in the industrial production of steel, components are mixed from raw materials such as ores and scraps and various factors in the production process, and are allowable components within a range not adversely affecting the present invention.
3. For coating film
As described above, the coating film is formed on at least a part of the surface of the base material. In addition, the total content of Cr, Ni, Cu, and Mo with respect to Fe is relatively increased in the coating film, and thus the acid resistance can be greatly improved.
Specifically, it is necessary to have a maximum Cr depth at which the Cr concentration becomes maximum in the coating film, and to satisfy the following formula (i) in the chemical composition at the maximum Cr depth. The position of the maximum Cr depth is not particularly limited, and may be present in the outermost layer of the coating film.
(Cr+Ni+Cu+Mo)/Fe≥1.0···(i)
Wherein each element symbol in the above formula represents the content (at%) of each element on the steel surface.
The coating film of the present invention has a structure including a surface layer side relatively rich in Cr and a base material side relatively rich in Ni and the like. That is, the minimum Cr depth at which the Cr concentration becomes minimum exists on the base material side of the maximum Cr depth.
Further, the chemical composition at the maximum Cr depth preferably satisfies the following formula (ii), and the chemical composition at the minimum Cr depth preferably satisfies the following formula (iii).
Cr/(Ni+Cu+Mo)≥1.0···(ii)
Cr/(Ni+Cu+Mo)<1.0···(iii)
Wherein each element symbol in the above formula represents the content (at%) of each element.
The thickness of the coating is not particularly limited, but is preferably in the range of 2 to 10nm, for example. When the thickness of the coating film is less than 2nm, the sulfuric acid corrosion resistance may not be sufficiently obtained. On the other hand, if the thickness of the coating exceeds 10nm, there is a concern that unevenness in the composition of the coating and peeling of the coating are likely to occur.
In the present invention, the chemical composition of the coating film is measured by depth analysis using X-ray photoelectron spectroscopy (XPS). By the above depth analysis, the concentration curve of each element was derived as the ratio (at%) of the components other than O, C and N. Then, the maximum Cr depth and the minimum Cr depth are determined, the concentrations of the respective elements at the depths are obtained, and the above-described equations (i) to (iii) are calculated from these values.
The thickness of the coating was determined from the O (oxygen) concentration curve. Specifically, the position of 1/3, which is the maximum concentration of O, is determined as the boundary between the coating and the base material, and the length from the surface of the coating to the boundary is defined as the thickness of the coating. The composition and thickness of the coating film are preferably measured at a plurality of locations, and the average value is used.
4. Manufacturing method
The conditions for producing the austenitic stainless steel of the present invention are not particularly limited, and for example, the austenitic stainless steel can be produced by subjecting a steel blank having the above chemical composition to heat treatment and acid treatment under the following conditions.
< Heat treatment Process >
The steel blank is first subjected to a heat treatment at a temperature of 1060 to 1140 ℃ for 60 to 600 seconds. Thereby, an oxide film mainly containing Fe is formed on the surface of the steel billet. When the heat treatment temperature is less than 1060 ℃, the formation of the Fe oxide film becomes insufficient. On the other hand, if the heat treatment temperature exceeds 1140 ℃, the crystal grains of the base material become coarse, and the diffusion of Fe becomes small, so that the Fe oxide film becomes uneven, and the film peeling easily occurs. As a result, in any of the above cases, the enrichment of Cr, Ni, Cu and Mo hardly occurs.
< acid treatment Process >
Subsequently, the steel billet is subjected to the heat treatment. In the acid treatment step, the Fe component is preferentially melted, so that Cr, Ni, Cu, and Mo can be concentrated on the steel surface. In order to preferentially melt the Fe component, it is preferable that HNO be dissolved at 30 to 50 ℃ and 5 to 8 vol%3And 5 to 8 vol% HF in nitric fluoride for 1 to 5 hours.
The present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.
Examples
Steels (steels Nos. 1 to 11) having chemical compositions shown in Table 1 were melted in a 3.5-t VIM melting furnace, and hot forging, hot extrusion and cold drawing were carried out by a usual method to prepare steel pipe blanks having an outer diameter of 75mm and a wall thickness of 3 mm. Thereafter, test Nos. 1 to 17 and 19 to 28 were subjected to heat treatment and acid treatment under the conditions shown in Table 2 to prepare austenitic stainless steel pipes. In addition, in test No.18, heat treatment and acid treatment were performed under the same conditions as in test No.3, and then the surface was polished.
[ Table 1]
Figure BDA0001966598070000131
[ Table 2]
TABLE 2
Figure BDA0001966598070000141
Next, the chemical composition and thickness of the coating film formed on the surface of each steel pipe were measured by XPS depth analysis. Specifically, the concentration curve of each element is derived as the ratio (at%) of the components other than O, C and N, the maximum Cr depth and the minimum Cr depth are determined, and the concentration of each element at the depth is determined. Then, the above-mentioned expressions (i) to (iii) are calculated from these values. In the present example, the maximum Cr depth was present in the outermost layer of the coating film in the examples other than test No.18, and the minimum Cr depth was present closer to the base material side than the maximum Cr depth in all the examples.
The thickness of the coating was determined from the O (oxygen) concentration curve. Specifically, the position of 1/3, which is the maximum concentration of O, is determined as the boundary between the coating and the base material, and the length from the surface of the coating to the boundary is defined as the thickness of the coating.
Further, in order to evaluate the corrosion resistance to sulfuric acid, a corrosion test in a sulfuric acid environment was performed. The corrosion test was performed by immersing each steel pipe in a solution having a temperature of 100 ℃ and a sulfuric acid concentration of 70%. Then, the corrosion loss after 8 hours of immersion was measured, and the corrosion rate per unit area was calculated. In the present invention, the etching rate is 1.00 g/(m)2H) or less, it is judged that the corrosion resistance to sulfuric acid is excellent.
These results are shown in Table 3.
[ Table 3]
Figure BDA0001966598070000161
As is clear from Table 3, in test Nos. 1, 2 and 14 to 17, which were not suitable for the production conditions, and test No.18, which was a polished surface, the coating film was not enriched with Cr, Ni, Cu and Mo, and therefore, the corrosion rate was high, and the sulfuric acid corrosion resistance was poor. Similarly, in test No.28 in which the Cu content in the base material was outside the limits of the present invention, the acid resistance due to Cu could not be obtained, and the enrichment of Cr, Ni, Cu, and Mo in the coating film was insufficient, resulting in poor sulfuric acid corrosion resistance.
On the other hand, in the test Nos. 3 to 13 and 19 to 27 which satisfy the limitations of the present invention and in which Cr, Ni, Cu and Mo were concentrated in the coating film, the corrosion rate was 1.00 g/(m)2H) or less, the corrosion resistance to sulfuric acid is excellent.
Industrial applicability
According to the present invention, an austenitic stainless steel having excellent acid resistance in an environment where sulfuric acid of high concentration is condensed can be obtained. Therefore, the austenitic stainless steel of the present invention can be used for various members such as heat exchangers, flues, and chimneys used in thermal power generation and industrial boilers, members for flue gas desulfurization devices used in various industries, and structural members used in facilities used in a sulfuric acid environment.

Claims (3)

1. An austenitic stainless steel, comprising: a base material and a coating film formed on at least a part of a surface of the base material,
the chemical composition of the base material is calculated by mass percent
C: less than 0.05 percent of,
Si: less than 1.0 percent,
Mn: less than 2.0 percent,
P: less than 0.040%,
S: less than 0.010%,
O: less than 0.020%,
N: less than 0.050%,
Ni：12.0～27.0％、
Cr: more than 15.0 percent and less than 20.0 percent,
Cu: more than 3.5% and not more than 8.0%,
Mo: more than 2.0% and not more than 5.0%,
Co: less than 0.05 percent of,
Sn: less than 0.05 percent of,
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％、
And the balance: fe and impurities in the iron-based alloy, and the impurities,
the chemical composition at the maximum Cr depth at which the Cr concentration of the coating film becomes maximum satisfies the following formula (i),
(Cr+Ni+Cu+Mo)/Fe≥1.0···(i)
wherein each element symbol in the formula represents the content of each element in at%.
2. The austenitic stainless steel according to claim 1, wherein the chemical composition of the base material contains, in mass%, a chemical composition selected from the group consisting of
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 to 0.01%, and
REM：0.0005～0.01％
1 or more of them.
3. The austenitic stainless steel according to claim 1 or claim 2, wherein a minimum Cr depth at which the Cr concentration of the coating film becomes minimum is present on the parent material side than the maximum Cr depth,
the chemical composition at the maximum Cr depth satisfies the following formula (ii), and the chemical composition at the minimum Cr depth satisfies the following formula (iii),
Cr/(Ni+Cu+Mo)≥1.0···(ii)
Cr/(Ni+Cu+Mo)<1.0···(iii)
wherein each element symbol in the formula represents the content of each element in at%.
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