JPH11158584A - Austenitic stainless steel excellent in sulfuric acid corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel excellent in corrosion resistance in the environment where sulfuric acid of high concentration is solidified. SOLUTION: This austenitic stainless steel has a composition consisting of, by weight, <=0.05% C, >1.0-3.0% Si, 0.1-2.0% Mn, 12-27% Ni, 16-26% Cr, >3.0-8.0% Cu, >2.0-5.0% Mo, <=0.5% Al, <0.05% N, <=0.04% P, <=0.005% S, <=0.01% Ca, <=0.01% B, <=0.01%, in total, of rare earth elements, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an austenitic stainless steel having excellent resistance to sulfuric acid corrosion, which is a problem in heat exchanger tubes, stacks, and chimneys used in thermal power generation and industrial boilers. It is about.

[0002]

2. Description of the Related Art So-called "fossil fuels" such as oil and coal used as boiler fuels for thermal power generation and industrial use contain sulfur (S). Therefore, when fossil fuels burn, sulfur oxides (SO _x ) are generated in the exhaust gas. 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] Therefore, in order to prevent dew condensation of sulfuric acid, in a heat exchanger used in an exhaust gas system, the temperature of the exhaust gas is maintained at a high temperature of 150 ° C or more so that sulfuric acid does not form dew on the surface of the member. .

However, in view of the recent increase in energy demand and effective use of energy, there has been a movement to recover heat energy as effectively as possible, for example, by lowering the temperature of exhaust gas from a heat exchanger. And resistant materials (materials with excellent sulfuric acid corrosion resistance)
Has come to be required.

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

On the other hand, anticorrosion technology (vol. 26 (1977)
As described in (Year) pp. 731 to 740), corrosion by sulfuric acid increases in a region where the temperature is lower by 20 ° C. to 60 ° C. than the dew point of sulfuric acid. This is because the amount of condensed sulfuric acid is small near the dew point. For this reason, when the exhaust gas temperature is not maintained at 150 ° C. or higher, generally,
In terms of temperature, around 100 ° C. is the region where corrosion resistance is most required, and here, the concentration of sulfuric acid is about 70%. However, in this region, not only general-purpose stainless steel but also low-alloy steel has a large amount of corrosion and cannot be used.

For a member in a sulfuric acid environment, a specific corrosion-resistant material may be used.
It has been proposed in JP 3860, JP-A-2-170946, JP-A-4-346638 and JP-A-5-156410.

Japanese Unexamined Patent Publication No. 56-93860 discloses that, at a temperature of about 100 ° C. and a sulfuric acid environment having a concentration of 95% or more, excellent corrosion resistance in a sulfuric acid environment, Cr: 18.0 to 29.0%, N
i: 20.0 to 45%, Mo: 4.0 to 9.0%, S
i: 1.5 to 5.0%, Cu: 0.5 to 3.0%, M
A "sulfuric acid corrosion resistant alloy" having a chemical composition of n: 2.0% or less and C: 0.10% or less is disclosed.

However, the steel proposed in this publication is Cu
Since the content is low, for example, the corrosion resistance in an environment where the concentration of sulfuric acid is about 70% at around 100 ° C. is not always sufficient.

JP-A-2-170946 discloses that C:
0.004 to 0.05%, Si: 5% or less, Mn: 2%
Hereinafter, Cr: 18 to 25%, Ni: 14 to 24%, M
o: 1 to 4.5%, Cu: 0.5 to 2.0%, Al:
0.05% or less, N: P based on 0.01 to 0.3%,
"High alloy stainless steel for chimneys / flue stacks and desulfurization equipment with excellent corrosion resistance" has been proposed in which the contents of S and O, the overall corrosion resistance index and the crevice corrosion resistance index are regulated.

[0011] The stainless steel described in the above-mentioned publication is indeed 1000 ppm of Fe ³⁺ and 1000 ppm in 50% sulfuric acid.
ppm of Cl ^- is a corrosion resistance in an environment with added are excellent, because of a low 0.5 to 2.0 wt% Cu content, for example, the concentration of sulfuric acid already 100 ° C. vicinity mentioned It is difficult to say that the corrosion resistance under an environment of about 70% is sufficient.

Japanese Patent Application Laid-Open No. 4-34638 discloses that C: 0.050% or less, Si: 1.00% or less, M
n: 2.00% or less, P: 0.050% or less, S: 0.
0050% or less, Ni: 8.0 to 30%, Cr: 15 to
28%, Mo: more than 2% and 7% or less, Cu: more than 2% and 5% or less, N: 0.05 to 0.35%, B: 0.001
Contains more than 5% and 0.010% or less, and 60 ppm of O
A "sulfuric acid dew-point corrosion resistant stainless steel excellent in hot workability", in which the contents of Cu, Mo, B and O are specified below, is disclosed.

The stainless steel described in this publication has a content of 0.05
An attempt is made to stabilize the austenitic structure and ensure corrosion resistance by adding N by weight or more. However, as a result of the study of the present inventors, when N is contained in an amount of 0.05% by weight or more, the corrosion resistance of the austenitic stainless steel to which Cu, Cr and Mo are added in combination may be lowered. It became clear. Furthermore, N
When the content was 0.05% by weight or more, it was also found that when the Cu content was increased in order to increase the corrosion resistance, the hot workability in a temperature range below 1000 ° C. was significantly reduced.

JP-A-5-156410 discloses that, by weight%, C: 0.04% or less, Si: 5 to 7%, Mn: 2
%, Cr: 15 to 25%, Ni: 4 to 24%, W:
A "high-temperature, high-concentration stainless steel for sulfuric acid" having a chemical composition of 0.5 to 3% is disclosed.

However, since the stainless steel proposed in this publication does not contain Cu, for example, the aforementioned 100 ° C.
Corrosion resistance in an environment where the concentration of sulfuric acid is about 70% in the vicinity is not always sufficient.

[0016]

SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust gas system member such as a boiler for thermal power generation or an industrial boiler which has excellent corrosion resistance in an environment in which high-concentration sulfuric acid condenses (sulfuric acid dew point environment). An object of the present invention is to provide an austenitic stainless steel that can be used for members such as a heat exchanger, a flue, and a chimney.

The "environment in which high-concentration sulfuric acid condenses" in the following description of the present specification refers to an environment in which sulfuric acid having a concentration of "40-70%" dew at a temperature of "50-100 ° C". As described above, corrosion due to sulfuric acid is greatest in a temperature range of 20 ° C. to 60 ° C. lower than the dew point of sulfuric acid. Therefore, in the present invention, the object of the present invention is to ensure corrosion resistance particularly in a sulfuric acid environment having a concentration of about 70% at around 100 ° C. where corrosion is the highest in the above environment.

[0018]

The gist of the present invention resides in the following austenitic stainless steel excellent in sulfuric acid corrosion resistance.

That is, "in terms of% by weight, C: 0.05% or less, Si: more than 1.0% and 3.0% or less, Mn: 0.
1 to 2.0%, Ni: 12 to 27%, Cr: 16 to 26
%, Cu: more than 3.0% and 8.0% or less, Mo: 2.
More than 0% and 5.0% or less, Al: 0.5% or less, N:
Less than 0.05%, P: 0.04% or less, S: 0.005
%, Ca: 0.01% or less, B: 0.01% or less,
Rare earth element: 0.01% or less in total, the balance being an austenitic stainless steel excellent in sulfuric acid corrosion resistance comprising Fe and unavoidable impurities. "

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors conducted a corrosion resistance test on a wide range of concentrations of sulfuric acid in order to ensure that Ni-Cr austenitic stainless steel has corrosion resistance in an environment where high concentrations of sulfuric acid congeal. The effects of alloying elements were studied in detail. As a result, it has been found that corrosion resistance is remarkably improved if Cu is contained in an amount exceeding 3% by weight and an appropriate amount of Si and Mo is added in combination.

Sulfuric acid having a concentration of 90% or more has a strong oxidizing property. Therefore, it is well known that in sulfuric acid having a concentration of 90% or more, Si forms an SiO ₂ film and is an effective element for corrosion resistance. However, in sulfuric acid at a concentration of less than 90%, there is not sufficient oxidizing property as a solution environment. For this reason, conventionally, it has been considered that the addition of Si to a steel used in sulfuric acid having a concentration of less than 90% has little effect on corrosion resistance and only deteriorates hot workability.

However, according to the study of the present inventors, it has been found that Cu, which electrochemically suppresses anode active dissolution and suppresses the generation of hydrogen as a cathode reaction, in a weight percentage of Cu.
It has been found that the corrosion resistance can be improved by adding more than 3% and adding an appropriate amount of Si together with Mo.

On the other hand, the deterioration of the hot workability due to the addition of Si can be suppressed by limiting the contents of N and S. Further, in order to enhance the hot workability, the addition of Ca, B or a rare earth element can be considered. It turned out to be good.

The present invention has been completed based on the above findings.

Hereinafter, the present invention will be described in detail. In addition, “%” of the component content means “% by weight”.

C: not more than 0.05% C has the effect of increasing the strength, but is combined with Cr to form Cr carbide at the grain boundaries, which lowers the intergranular corrosion resistance. The following is assumed. If it is necessary to increase the strength, it may be contained in an amount exceeding 0.03% to 0.05%.
Is desirably 0.03% or less.

Si: more than 1.0% and not more than 3.0% Si, when added in combination with Cu and Mo described later, has an effect of increasing the corrosion resistance in sulfuric acid having a concentration of less than 90%.
However, when the content is 1.0% or less, the effect of addition is poor. On the other hand, when the Cu content is large, the Si content is 3.0.
%, The hot workability is extremely deteriorated.
Therefore, the content of Si exceeds 1.0% to 3.0%.
It was as follows.

Mn: 0.1 to 2.0% Mn has the effect of fixing S to improve hot workability and stabilizing austenite. However, if the content is less than 0.1%, the effect of addition is poor.
On the other hand, if the content exceeds 2.0%, the effect is saturated and the cost is increased. Therefore, the content of Mn is set to 0.1 to 2.0%.

Ni: 12 to 27% Ni has an effect of stabilizing austenite.
However, if the content is less than 12%, a sufficient effect cannot be obtained. Ni also has an effect of enhancing corrosion resistance in a sulfuric acid environment, and this effect is particularly enhanced when its content is 14% or more. However, since Ni is an expensive element, if its content exceeds 27%, the cost becomes extremely high and the economy is lacking. Therefore, the content of Ni is 12 to 27%.
And

Cr: 16 to 26% Cr is an element effective for ensuring the corrosion resistance of austenitic stainless steel. Particularly, in an austenitic stainless steel in which N is restricted to the content described below, 16%
When the above-mentioned Cr is contained together with the above-mentioned amounts of Si and the amounts of Cu and Mo described later, good corrosion resistance can be ensured in an "environment in which high-concentration sulfuric acid condenses". However, if a large amount of Cr is contained, the corrosion resistance in the above environment may be deteriorated even in the case of an austenitic stainless steel in which the N content is regulated and Si, Cu, and Mo are added in combination. . Further, the workability is also reduced. In particular, if the Cr content exceeds 26%, the corrosion resistance of the austenitic stainless steel in the above environment may significantly deteriorate. Therefore, the content of Cr is set to 16 to 26%. From the viewpoint of hot workability, the content of Cr is preferably set to 23% or less.

Cu: more than 3.0% and not more than 8.0% Cu is an essential element for ensuring corrosion resistance in a sulfuric acid environment. More than 3.0% of Cu is added to the above amount of Si, C
By containing it together with r and the amount of Mo described below, good corrosion resistance can be imparted to the austenitic stainless steel in which N is regulated to the content described below in an “environment in which high-concentration sulfuric acid condenses”. Since the effect of improving corrosion resistance increases as the content of Cu added in combination with Si, Cr and Mo increases, the Cu content is preferably set to 4.0% or more. A more preferred content of Cu is 5.0% or more. Incidentally, by increasing the content of Cu, the corrosion resistance in the above environment is improved, but the hot workability is reduced, and particularly, C
If the content of u exceeds 8.0%, significant deterioration in hot workability occurs even if N is limited to the content described below. Therefore, the content of Cu is set to more than 3.0% and not more than 8.0%.

Mo: more than 2.0% and not more than 5.0% Mo is an element effective in ensuring the corrosion resistance of austenitic stainless steel. In particular, when Mo in an amount exceeding 2.0% is contained together with the above-mentioned amounts of Si, Cu and Cr, in the above-mentioned "environment in which high-concentration sulfuric acid solidifies", N is regulated to the content described below. Good corrosion resistance can be imparted to austenitic stainless steel.
However, when a large amount of Mo is contained, the hot workability deteriorates. In particular, when the content of Mo exceeds 5.0%, the hot workability remarkably deteriorates even if N is limited to the content described below. Is generated. Therefore, when the content of Mo exceeds 2.0%;
0% or less.

Al: 0.5% or less If the content of Al exceeds 0.5%, the hot workability deteriorates even in an austenitic stainless steel in which N is limited to the content described below. Therefore, the Al content is set to 0.5% or less. The lower limit of the Al content may be in the range of unavoidable impurities. However, since Al has a deoxidizing effect, it may be positively added. The content of Al when exerting a deoxidizing action is preferably 0.01% or more.

N: less than 0.05% N is an important element in the present invention. Conventionally, N has been positively added for the purpose of stabilizing the austenite structure and improving corrosion resistance. However, in the "environment in which high-concentration sulfuric acid condenses,"
Is 0.05% or more, the content of Si exceeds 1.0% and the content of C exceeds 3.0% for the purpose of enhancing corrosion resistance.
The corrosion resistance of the austenitic stainless steel containing u, Mo exceeding 2.0% and 16-26% Cr is rather deteriorated. Further, even when the upper limits of the contents of Si, Cu, and Mo are set to 3.0%, 8.0%, and 5.0%, respectively, when the content of N becomes 0.05% or more, hot Workability will be reduced. Therefore, in order to impart corrosion resistance and hot workability to an austenitic stainless steel in an "environment in which high-concentration sulfuric acid solidifies", the N content is set to less than 0.05%. Note that the lower the N content, the better.

P: not more than 0.04% Since P deteriorates hot workability and corrosion resistance, its content is preferably as low as possible. In particular, if it exceeds 0.04%, "an environment in which high-concentration sulfuric acid condenses" , Corrosion resistance is significantly deteriorated. Therefore, the content of P is set to 0.04% or less.

S: 0.005% or less S is an element that deteriorates hot workability, and its content is preferably as small as possible. In particular, for the purpose of enhancing corrosion resistance, more than 1.0% of Si and more than 3.0% of C
u, in an austenitic stainless steel containing Mo exceeding 2.0%, the S content is 0.005%.
If it exceeds 300, hot workability will be significantly reduced. Therefore, the content of S is set to 0.005% or less. Note that S
The content is preferably 0.003% or less.

Ca: 0.01% or less Ca may not be added. If added, it has the effect of bonding with S and suppressing a decrease in hot workability. In order to surely obtain this effect, the content of Ca is preferably set to 0.0005% or more. A more preferable lower limit of the Ca content is 0.001%. However, its content is 0.01%
If it exceeds, the cleanliness of the steel decreases, which causes flaws during hot production. Therefore, the content of Ca is set to 0.1.
01% or less.

B: 0.01% or less B may not be added. If added, it has the effect of improving hot workability. In order to surely obtain this effect, the content of B is preferably 0.0005% or more. A more preferred lower limit of the B content is 0.001%. However, the addition of a large amount of B promotes the precipitation of the Cr-B compound at the grain boundaries, leading to deterioration of corrosion resistance. In particular, when the content of B is 0.1.
If it exceeds 01%, the corrosion resistance is remarkably deteriorated. Therefore, the content of B is set to 0.01% or less.

Rare earth elements: 0.01% or less in total Rare earth elements need not be added. If added, it has the effect of improving hot workability. In order to surely obtain this effect, the content of the rare earth element is preferably set to 0.0005% or more in total. However, the content is 0.1% in total.
If it exceeds 01%, the degree of cleanliness of the steel is reduced, which causes flaws during hot production. Therefore, the content of the rare earth element is set to 0.01% or less in total.

[0040]

EXAMPLE An austenitic stainless steel having the chemical composition shown in Table 1 was melted using a high-frequency vacuum melting furnace of 20 kg. Steels 1 to 9 in Table 1 are steels of the examples of the present invention.
Nos. 0 to 15 are steels of comparative examples in which any of the components is out of the range of the content specified in the present invention. In this embodiment, the rare earth element is a composite addition of La and Ce.

[0041]

[Table 1]

Next, these steel ingots were heated to 1200 ° C., and then hot forged at a temperature of 900 ° C. or more.
The plate was finished to have a thickness of 30 mm, a width of 100 mm and a length of 300 mm.

The occurrence of cracks in the hot forged material was visually observed from the surface in the width direction, and the maximum crack length was measured as "forging crack length" and used as an index of hot workability.

Next, the part where the forging crack occurred was prepared, heated to 1200 ° C., and then hot-rolled to a steel sheet having a thickness of 8 mm. From this steel sheet, a corrosion test piece having a thickness of 3 mm, a width of 10 mm and a length of 40 mm was prepared by machining, and subjected to a corrosion test in a sulfuric acid environment.

In the above corrosion test, in order to simulate dew-point corrosion in which sulfuric acid is condensed on the surface of a steel member, the steel was immersed in a solution having a temperature of 100 ° C. and a sulfuric acid concentration of 70% for 5 minutes.
The holding in the atmosphere at 00 ° C. for one hour was repeated five times. The corrosion weight loss after this test was measured, and the corrosion rate per unit area was calculated to evaluate the sulfuric acid corrosion resistance.

The target value of the forging crack length, which is an index of hot workability, is 3 mm or less, and the target value of sulfuric acid corrosion resistance is 0.1 mm.
It was 10 g / (m ² · h) or less.

Table 2 shows the results of the investigation on the sulfuric acid corrosion resistance and hot workability.

[0048]

[Table 2]

From Table 2, it can be seen that the steels of the present invention have the desired hot workability and sulfuric acid corrosion resistance. Even in the steels of the present invention, in steels 5 to 9 to which Ca, B and rare earth elements are added, the forging crack length is extremely small at 0.5 mm or less, and it is clear that the hot workability is extremely good. .

On the other hand, it is clear that steel 12 and steel 13 having a Si content exceeding the specification of the present invention have a large forging crack length and are inferior in hot workability. Steel 15 containing 0.064% of N, which deviates from the definition of the present invention, also has a large forging crack length and is inferior in hot workability. Further, the steel 14 having an S content exceeding 0.005%, which is the prescribed upper limit of the present invention, also has a large forging crack length and is inferior in hot workability.

With respect to the sulfuric acid corrosion resistance, the corrosion rates of steel 10 and steel 12 having a Si content lower than the prescribed value of the present invention, and steel 11 and steel 12 having a Cu content lower than the prescribed value of the present invention are high. Not corrosive.

[0052]

The austenitic stainless steel of the present invention has excellent corrosion resistance in an environment in which high-concentration sulfuric acid is condensed. It can be used for members such as roads and chimneys.

Claims (1)

[Claims] (1) In weight%, C: 0.05% or less, Si:
More than 1.0% and 3.0% or less, Mn: 0.1 to 2.0
%, Ni: 12 to 27%, Cr: 16 to 26%, Cu:
More than 3.0% to 8.0% or less, Mo: more than 2.0% to 5.0% or less, Al: 0.5% or less, N: 0.05%
, P: 0.04% or less, S: 0.005% or less, C
a: 0.01% or less, B: 0.01% or less, rare earth element: 0.01% or less in total, the balance being Fe and inevitable impurities, austenitic stainless steel excellent in sulfuric acid corrosion resistance.