JP6724991B2 - Austenitic stainless steel - Google Patents

Description

The present invention relates to austenitic stainless steel, and more particularly to austenitic stainless steel having excellent acid resistance.

Sulfur (S) is contained in so-called "fossil fuels" such as oil and coal used as boiler fuels for thermal power generation or industrial use. Therefore, when the fossil fuel burns, sulfur oxides (SO _x ) are generated in the exhaust gas. When the temperature of the exhaust gas decreases, SO _x reacts with the water content in the gas to form sulfuric acid, which condenses on the surface of the member having a low temperature below the dew point temperature, which causes sulfuric acid dew point corrosion.

Similarly, in a flue gas desulfurization apparatus used in various industries, when a gas containing SO _x flows, sulfuric acid dew point corrosion occurs when the temperature of the gas decreases. Hereinafter, in the present specification, a gas containing SO _x will be described as an exhaust gas for the sake of simplicity.

Because of the above phenomenon, in a heat exchanger used for an exhaust gas system, the exhaust gas temperature is kept at a high temperature of 150° C. or higher so that sulfuric acid does not condense on the surface of the member.

However, from the viewpoint of recent increase in energy demand and effective use of energy, in order to recover heat energy as effectively as possible, for example, there is a movement to lower the exhaust gas temperature from the heat exchanger to below the dew point of sulfuric acid. Therefore, a material having resistance has come to be demanded.

When the exhaust gas temperature is not maintained at 150° C. or higher, sulfuric acid having a high concentration of about 80% is condensed on the surface of the member from the exhaust gas having a general composition in the temperature range of about 140° C. In such an environment, so-called "low alloy steel" has been used as steel for various members. This is because the low-alloy steel has higher corrosion resistance than the general-purpose stainless steel against the high-temperature and high-concentration sulfuric acid as described above.

On the other hand, as described in Non-Patent Document 1, the amount of sulfuric acid that condenses in the region where the temperature is 20 to 60° C. lower than the dew point of sulfuric acid becomes the largest, so that the corrosion by sulfuric acid becomes large. Therefore, when the exhaust gas temperature is not maintained at 150° C. or higher, generally, the temperature is in the region where the highest corrosion resistance is required at a temperature near 100° C., and the concentration of sulfuric acid is about 70% here. However, in this region, not only general-purpose stainless steel but also low-alloy steel cannot be used due to its large amount of corrosion.

It has been proposed so far that a specific corrosion-resistant material may be used for a member in a sulfuric acid environment. For example, Patent Document 1 discloses a sulfuric acid dew-point corrosion-resistant stainless steel excellent in hot workability. Is disclosed.

Further, Patent Document 2 discloses an austenitic stainless steel having excellent resistance to sulfuric acid corrosion and also excellent workability.

JP-A-4-346638 Patent No. 3294282

Hiroo Nagano, "Sulfuric Acid Dew Point Corrosion", Anticorrosion Technology, 1977, Vol. 26, No. 12, p. 731-740

The stainless steel described in Patent Document 1 contains 0.05% by weight or more of N (nitrogen) in an attempt to stabilize the austenite structure and ensure corrosion resistance. However, when N is contained in an amount of 0.05% by weight or more, the sulfuric acid corrosion resistance of the austenitic stainless steel to which Cu, Cr and Mo are added is rather deteriorated. Further, when the N content is 0.05% by weight or more, if the Cu content is increased in order to improve the sulfuric acid corrosion resistance, the hot workability in the temperature range lower than 1000° C. remarkably decreases. There is a problem of becoming.

Further, the austenitic stainless steel described in Patent Document 2 has excellent sulfuric acid corrosion resistance and workability. However, there is still room for improvement in sulfuric acid corrosion resistance.

An object of the present invention is to solve the above problems and to provide an austenitic stainless steel having excellent acid resistance in an environment where a high concentration of sulfuric acid is condensed.

In the following description, the "environment in which high-concentration sulfuric acid condenses" means an environment in which 40-70% concentration of sulfuric acid condenses at a temperature of 50 to 100°C.

The present invention has been made in order to solve the above problems, and has as its gist the following austenitic stainless steels.

(1) An austenitic stainless steel comprising a base material and a film formed on at least a part of the surface of the base material,
The chemical composition of the base material is% by 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: more than 2.0% and 5.0% or less,
Co: 0.05% or less,
Sn: 0.05% or less,
V: 0 to 0.5%,
Nb: 0 to 1.0%,
Ti: 0 to 0.5%,
W: 0-5.0%,
Zr: 0 to 1.0%,
Al: 0 to 0.5%,
Ca: 0 to 0.01%,
B: 0 to 0.01%,
REM: 0 to 0.01%,
Balance: Fe and impurities,
The chemical composition at the maximum Cr depth that maximizes the Cr concentration of the coating satisfies the following formula (i),
Austenitic stainless steel.
(Cr+Ni+Cu+Mo)/Fe≧1.0 (i)
However, each element symbol in the above formula represents the content (at %) of each element.

(2) The chemical composition of the base material is% by mass,
V: 0.01 to 0.5%,
Nb: 0.02-1.0%,
Ti: 0.01 to 0.5%,
W: 0.1-5.0%,
Zr: 0.02-1.0%,
Al: 0.01 to 0.5%,
Ca: 0.0005-0.01%,
B: 0.0005 to 0.01%, and
REM: 0.0005 to 0.01%,
Containing one or more selected from,
The austenitic stainless steel according to (1) above.

(3) The minimum Cr depth at which the Cr concentration of the coating is minimum exists on the base metal side with respect to 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),
The austenitic stainless steel according to (1) or (2) above.
Cr/(Ni+Cu+Mo)≧1.0 (ii)
Cr/(Ni+Cu+Mo)<1.0 (iii)
However, each element symbol in the above formula represents the content (at %) of each element.

According to the present invention, an austenitic stainless steel having excellent acid resistance can be obtained in an environment in which a high concentration of sulfuric acid is condensed.

The present inventors have earnestly studied a method for further improving sulfuric acid corrosion resistance based on the austenitic stainless steel described in Patent Document 2, and have obtained the following findings.

In order to improve the sulfuric acid corrosion resistance, the composition of the film formed on the surface of the base material in contact with high concentration sulfuric acid is important. By increasing the total content of Cr, Ni, Cu and Mo in the film relative to Fe, it becomes possible to significantly improve the acid resistance.

In addition, after heat-treating the steel under predetermined conditions to form an oxide film mainly composed of Fe on the surface, acid treatment is performed to preferentially dissolve the Fe component, so that the Cr content in the film is increased. , Ni, Cu and Mo have been found to be able to be concentrated.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. Configuration The austenitic stainless steel according to the present invention includes a base material and a film formed on at least a part of the surface of the base material. Each of the base material and the film will be described in detail below.

2. Base Material The chemical composition of the base material will be described in detail. The reasons for limiting each element are as follows. In the following description, “%” regarding the content means “mass %”.

C: 0.05% or less C is an element having an action of increasing strength. However, they combine with Cr to form Cr carbides at the grain boundaries, which lowers the intergranular corrosion resistance. Therefore, the C content is 0.05% or less. When it is necessary to increase the strength, it is preferable that the content be more than 0.03%. On the other hand, when priority is given to ensuring corrosion resistance, the C content is preferably low, and is preferably 0.03% or less. The lower limit need not be set in particular, but in order to obtain the above effects, the C content is preferably 0.01% or more.

Si: 1.0% or less Si is an element having a deoxidizing effect. However, when its content exceeds 1.0%, it promotes the deterioration of hot workability, and in combination with the increase of Cu content, it becomes extremely difficult to process the product on an industrial scale. Therefore, the Si content is set to 1.0% or less. The Si content is preferably 0.6% or less. The lower limit is not particularly set because Si does not necessarily have to be contained, but in order to obtain the above effects, the Si content is preferably 0.05% or more. Further, when the Al content is made extremely low for the purpose of improving hot workability, it is preferable to add 0.1% or more of Si to sufficiently perform the deoxidizing action.

Mn: 2.0% or less Mn has a function of fixing S to improve hot workability and stabilizing an austenite phase. However, even if Mn is contained in an amount exceeding 2.0%, the effect is saturated and the cost is increased. Therefore, the Mn content is set to 2.0% or less. The Mn content is preferably 1.5% or less. Since Mn does not necessarily have to be contained, the lower limit is not particularly set, but in order to obtain the above effect, the Mn content is preferably 0.1% or more.

P: 0.040% or less P is contained in steel as an impurity and deteriorates hot workability and corrosion resistance. Therefore, its content is preferably as low as possible. In particular, when the P content exceeds 0.040%, the corrosion resistance is significantly deteriorated in the environment where a high concentration of sulfuric acid is condensed. Therefore, the P content is 0.040% or less.

S: 0.010% or less S is contained in the steel as an impurity and deteriorates the hot workability. Therefore, the S content is preferably as low as possible. In particular, if the S content exceeds 0.010%, the hot workability is significantly deteriorated. Therefore, the S content is set to 0.010% or less.

O: 0.020% or less O is contained in the steel as an impurity and deteriorates hot workability and ductility. Therefore, its content is preferably as low as possible. In particular, when the O content exceeds 0.020%, the hot workability and ductility are significantly deteriorated, so the O content is set to 0.020% or less.

N: Less than 0.050% N has conventionally been positively added for the purpose of stabilizing the austenite structure or increasing the resistance to local corrosion such as pitting or crevice corrosion. However, in an environment where a high concentration of sulfuric acid condenses, when the N content is 0.050% or more, Cu exceeding 3.5%, Mo exceeding 2.0% and 15.0% or more 20.0 %, the corrosion resistance of the austenitic stainless steel containing less than Cr will rather decrease. Further, even when the upper limits of the Cu and Mo contents are 8.0% and 5.0%, respectively, the hot workability is deteriorated when the N content is 0.050% or more. .. The N content is set to less than 0.050% in order to impart corrosion resistance and hot workability in an environment in which a high concentration of sulfuric acid is condensed to the austenitic stainless steel. The lower the N content, the better, and it is preferably 0.045% or less.

Ni: 12.0 to 27.0%
Ni has a function of stabilizing the austenite phase and also a function of enhancing corrosion resistance in an environment where a high concentration of sulfuric acid is condensed. In order to sufficiently secure such effects, it is necessary to contain Ni in an amount of 12.0% or more. However, the effect is saturated even if the content exceeds 27.0%. Furthermore, since Ni is an expensive element, the cost is extremely high and the economy is low. Therefore, the Ni content is set to 12.0 to 27.0%. In order to secure sufficient corrosion resistance in an environment where high-concentration sulfuric acid is condensed, it is preferable to contain Ni in an amount exceeding 15.0%, and Ni in an amount exceeding 20.0%. Is more preferable.

Cr: 15.0% or more and less than 20.0% Cr is an element effective for ensuring the corrosion resistance of austenitic stainless steel. Particularly, in the austenitic stainless steel in which N is regulated to the above content, if 15.0% or more of Cr, preferably 16.0% or more of Cr is contained together with the amounts of Cu and Mo described later, a high concentration is obtained. Good corrosion resistance can be ensured in an environment where sulfuric acid condenses. However, when Cr is excessively contained, the N content is lowered, and even in the case of the austenitic stainless steel in which Cu and Mo are added together, the corrosion resistance in the environment is rather deteriorated, and the workability is further deteriorated. Also decreases. In particular, when the Cr content is 20.0% or more, deterioration of corrosion resistance of the austenitic stainless steel in the environment becomes remarkable. Further, by setting the Cr content to be less than 20.0%, the hot workability of the austenitic stainless steel to which Cu and Mo are added in combination can be improved, and product processing on an industrial scale can be facilitated. It will be possible. Therefore, the Cr content is set to 15.0% or more and less than 20.0%.

Cu: more than 3.5% and 8.0% or less Cu is an essential element for ensuring corrosion resistance in a sulfuric acid environment. By containing more than 3.5% Cu together with the above-mentioned amount of Cr and the amount of Mo described later, it is preferable for the austenitic stainless steel containing N in the above-mentioned amount in an environment where a high concentration of sulfuric acid is condensed. Corrosion resistance can be imparted. Since the greater the content of Cu to be added together with Cu and Mo, the greater the effect of improving corrosion resistance, the Cu content is preferably set to an amount exceeding 4.0%. It should be noted that by increasing the Cu content, the corrosion resistance in the environment is improved, but the hot workability is deteriorated. Particularly, when the Cu content exceeds 8.0%, N is set to the above content. Remarkably deteriorates hot workability. Therefore, the Cu content is more than 3.5% and not more than 8.0%.

Mo: more than 2.0% and 5.0% or less 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 the amounts of Cr and Cu described above, good corrosion resistance is imparted to the austenitic stainless steel containing N in the above amount in an environment where high-concentration sulfuric acid is condensed. can do. However, when Mo is excessively contained, hot workability is deteriorated, and particularly when the Mo content exceeds 5.0%, the hot workability is remarkably deteriorated even when N is contained in the above range. Therefore, the Mo content is more than 2.0% and 5.0% or less. In order to secure sufficient corrosion resistance in an environment where high-concentration sulfuric acid is condensed, it is preferable to contain Mo in an amount exceeding 3.0%.

Co: 0.05% or less Co is an element contained in steel as an impurity. Co is an element effective for increasing the toughness of steel, but it is an expensive element, so it is not necessary to add it positively. Therefore, the Co content is 0.05% or less.

Sn: 0.05% or less Since Sn is contained in steel as an impurity and deteriorates hot workability, its content is preferably as low as possible. In particular, if the Sn content exceeds 0.05%, the hot workability is significantly deteriorated. Therefore, the Sn content is set to 0.05% or less.

V: 0.5% or less V has the effect of fixing C and increasing corrosion resistance, especially intergranular corrosion resistance, so V may be contained if necessary. However, when the content exceeds 0.5%, nitrides are generated even when the content of N is set to the above-mentioned amount, and the corrosion resistance is rather deteriorated, and further the hot workability is deteriorated. Therefore, the V content is 0.5% or less. In order to obtain the above effects, the V content is preferably 0.01% or more.

Nb: 0 to 1.0%
Nb has a function of fixing C and improving corrosion resistance, especially intergranular corrosion resistance, and thus may be contained if necessary. However, if the content exceeds 1.0%, even if the content of N is set to the above-mentioned content, a nitride is generated and the corrosion resistance is rather deteriorated, 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 to 0.5%
Ti, like Nb, has the effect of fixing C and increasing corrosion resistance, especially intergranular corrosion resistance, so Ti may be contained if necessary. However, when the content exceeds 0.5%, nitrides are generated even when the content of N is set to the above-mentioned amount, and the corrosion resistance is rather deteriorated, and further the hot workability is deteriorated. Therefore, the Ti content is 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 a function of enhancing the corrosion resistance in an environment in which a high concentration of sulfuric acid is condensed, so W may be contained if necessary. However, if the content exceeds 5.0%, the above effects are saturated, and the cost only 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 to 1.0%
Zr has a function of enhancing the corrosion resistance in an environment in which a high concentration of sulfuric acid is condensed, and thus may be contained if necessary. However, if the content exceeds 1.0%, the above effects are saturated, and the cost only 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 to 0.5%
Since Al has a deoxidizing action, it may be contained if the Si content is to be kept extremely low. However, if the content exceeds 0.5%, the hot workability deteriorates even with austenitic stainless steel containing N in the above content. Therefore, the Al content is 0.5% or less. The lower limit of the Al content is not particularly specified and may be in the range of impurities. However, in the case of suppressing the Si content to an extremely low level, it is preferable that the Si content is positively added to contain 0.02% or more to sufficiently perform the deoxidizing action. Even when Si is contained in an amount of 0.05% or more, the Al content is preferably 0.01% or more in order to sufficiently exert the deoxidizing effect.

Ca: 0 to 0.01%
Ca has an effect of suppressing deterioration of hot workability by combining with S, and thus may be contained if necessary. However, if its content exceeds 0.01%, the cleanliness of the steel decreases, causing defects during hot production. 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 to 0.01%
B has an effect of improving hot workability, and thus may be contained if necessary. However, excessive addition of B promotes precipitation of the Cr-B compound at the grain boundaries, leading to deterioration of corrosion resistance. In particular, if the B content exceeds 0.01%, the corrosion resistance is significantly deteriorated. Therefore, the B content is 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 to 0.01%
REM (rare earth element) has a function of enhancing hot workability, and thus may be contained if necessary. However, if its content exceeds 0.01%, the cleanliness of the steel decreases, causing defects during hot production. Therefore, the REM content is 0.01% or less. In order to obtain the above effects, the REM content is preferably 0.0005% or more.

Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements.

In the chemical composition of the base material of the austenitic stainless steel of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component that is mixed in by raw materials such as ore and scrap when manufacturing steel industrially, and various factors in the manufacturing process, and is allowed within a range that does not adversely affect the present invention. Means something.

3. About the coating As described above, the coating is formed on at least a part of the surface of the base material. Then, by increasing the total content of Cr, Ni, Cu and Mo in the film relative to Fe, it becomes possible to significantly improve the acid resistance.

Specifically, the maximum Cr depth that maximizes the Cr concentration exists in the film, and the chemical composition at the maximum Cr depth must satisfy the following formula (i). The position of the maximum Cr depth is not particularly limited and may be present in the outermost layer of the film.
(Cr+Ni+Cu+Mo)/Fe≧1.0 (i)
However, each element symbol in the above formula represents the content (at %) of each element on the steel surface.

The coating according to the present invention generally has a structure including a layer on the surface layer side in which Cr is relatively concentrated and a layer on the base material side in which Ni or the like is relatively concentrated. That is, the minimum Cr depth that minimizes the Cr concentration exists on the base metal side with respect to the above maximum Cr depth.

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)
However, each element symbol in the above formula represents the content (at %) of each element.

The thickness of the film is not particularly limited, but is preferably in the range of 2 to 10 nm, for example. If the thickness of the film is less than 2 nm, sulfuric acid corrosion resistance may not be sufficiently obtained. On the other hand, if the thickness of the film exceeds 10 nm, the composition of the film may be non-uniform and the film may be easily peeled off.

In the present invention, the chemical composition of the film is to be measured by depth analysis using X-ray photoelectric spectroscopy (XPS). By the above depth analysis, the concentration profile of each element is derived as a ratio (at %) to the components excluding O, C and N. Then, by specifying the maximum Cr depth and the minimum Cr depth, the concentration of each element at the depth is obtained, and the above equations (i) to (iii) are calculated from these values.

The thickness of the film is determined from the O (oxygen) concentration profile. Specifically, the position where the concentration of O is 1/3 of the maximum concentration is determined to be the boundary between the film and the base material, and the length from the film surface to the above boundary is the film thickness. .. It is desirable to measure the composition and thickness of the film at a plurality of points and use the average value thereof.

4. Manufacturing method There is no particular limitation on the manufacturing conditions of the austenitic stainless steel according to the present invention, for example, by subjecting the steel material having the above-described chemical composition to heat treatment and acid treatment under the conditions shown below, it is manufactured. be able to.

<Heat treatment process>
First, the steel material is subjected to a heat treatment of holding it in the temperature range of 1060 to 1140° C. for 60 to 600 seconds. As a result, an oxide film mainly containing Fe is formed on the surface of the steel material. If the heat treatment temperature is lower than 1060°C, the formation of the Fe oxide film will be insufficient. On the other hand, when the heat treatment temperature exceeds 1140° C., the crystal grains of the base material become coarse and Fe diffusion decreases, so that the Fe oxide film becomes non-uniform, and film peeling easily occurs. As a result, in any of the above cases, Cr, Ni, Cu, and Mo are less likely to be concentrated.

<Acid treatment step>
An acid treatment is applied to the steel material after the heat treatment. By preferentially dissolving the Fe component in the acid treatment step, it becomes possible to concentrate Cr, Ni, Cu and Mo on the steel surface. In order to preferentially dissolve the Fe component, it is preferable to immerse it in fluorinated nitric acid at 30 to 50° C., 5 to 8 volume% HNO ₃ and 5 to 8 volume% HF for 1 to 5 hours.

Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

Steels (steel Nos. 1 to 11) having the chemical compositions shown in Table 1 were melted using a 3.5t VIM melting furnace, and hot forged, hot extruded and cold drawn by a usual method. A steel pipe material having an outer diameter of 75 mm and a wall thickness of 3 mm was produced. After that, the test No. Regarding 1 to 17 and 19 to 28, heat treatment and acid treatment were performed under the conditions shown in Table 2 to obtain austenitic stainless steel pipes. In addition, the test No. For No. 18, test No. After heat treatment and acid treatment under the same conditions as in No. 3, the surface was polished.

Next, the chemical composition and thickness of the film formed on the surface of each steel pipe were measured by depth analysis using XPS. Specifically, the concentration profile of each element is derived as a ratio (at %) to the components excluding O, C, and N, the maximum Cr depth and the minimum Cr depth are specified, and then each element at that depth is determined. Was determined. Then, the above formulas (i) to (iii) were calculated from those values. In this example, the test No. In all the examples except 18, the maximum Cr depth was present in the outermost surface layer of the film, and in all the examples, the minimum Cr depth was present on the base metal side with respect to the maximum Cr depth.

The thickness of the film was determined from the O (oxygen) concentration profile. Specifically, the position where the concentration of O is 1/3 of the maximum concentration is determined to be the boundary between the film and the base material, and the length from the film surface to the above boundary is defined as the film thickness. ..

Furthermore, in order to evaluate the sulfuric acid corrosion resistance, a corrosion test was carried out in a sulfuric acid environment. The corrosion test was carried out by immersing each steel pipe in a solution having a temperature of 100° C. and a sulfuric acid concentration of 70%. Then, the corrosion weight loss after immersion for 8 hours was measured, and the corrosion rate per unit area was calculated. In the present invention, it was decided that the sulfuric acid corrosion resistance was excellent when the corrosion rate was 1.00 g/(m ² ·h) or less.

The results are also shown in Table 3.

As can be seen from Table 3, the test No. whose manufacturing conditions are inappropriate. Test Nos. 1 and 2 and 14 to 17 and polished skin. In No. 18, Cr, Ni, Cu and Mo were not concentrated in the film, so that the corrosion rate was high and the sulfuric acid corrosion resistance was inferior. Similarly, the test No. in which the Cu content in the base material is out of the regulation of the present invention. In No. 28, the acid resistance due to Cu was not obtained, and the concentration of Cr, Ni, Cu and Mo in the film was insufficient, so the sulfuric acid corrosion resistance was inferior.

On the other hand, Test No. which satisfies the requirements of the present invention and in which Cr, Ni, Cu and Mo are concentrated in the coating film. In 3 to 13 and 19 to 27, the corrosion rate was 1.00 g/(m ² ·h) or less, and the result was excellent in sulfuric acid corrosion resistance.

According to the present invention, an austenitic stainless steel having excellent acid resistance can be obtained in an environment in which a high concentration of sulfuric acid is condensed. Therefore, the austenitic stainless steel according to the present invention is used in heat exchangers used in thermal power generation or industrial boilers, flues and chimneys, and flue gas desulfurization equipment members used in various industries or sulfuric acid environments. It can be applied to various members such as structural members used in equipment.

Claims (3)

A base material and an austenitic stainless steel having a coating formed on at least a part of the surface of the base material,
The chemical composition of the base material is% by 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: more than 2.0% and 5.0% or less,
Co: 0.05% or less,
Sn: 0.05% or less,
V: 0 to 0.5%,
Nb: 0 to 1.0%,
Ti: 0 to 0.5%,
W: 0-5.0%,
Zr: 0 to 1.0%,
Al: 0 to 0.5%,
Ca: 0 to 0.01%,
B: 0 to 0.01%,
REM: 0 to 0.01%,
Balance: Fe and impurities,
The thickness of the film is 2 nm or more,
The chemical composition at the maximum Cr depth that maximizes the Cr concentration of the coating satisfies the following formula (i),
Austenitic stainless steel.
(Cr+Ni+Cu+Mo)/Fe≧1.0 (i)
However, each element symbol in the above formula represents the content (at %) of each element. The chemical composition of the base material is% by mass,
V: 0.01 to 0.5%,
Nb: 0.02-1.0%,
Ti: 0.01 to 0.5%,
W: 0.1-5.0%,
Zr: 0.02-1.0%,
Al: 0.01 to 0.5%,
Ca: 0.0005-0.01%,
B: 0.0005 to 0.01%, and
REM: 0.0005 to 0.01%,
Containing one or more selected from,
The austenitic stainless steel according to claim 1. The minimum Cr depth that minimizes the Cr concentration of the coating is present on the base metal side with respect to 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),
The austenitic stainless steel according to claim 1 or 2.
Cr/(Ni+Cu+Mo)≧1.0 (ii)
Cr/(Ni+Cu+Mo)<1.0 (iii)
However, each element symbol in the above formula represents the content (at %) of each element.