JPH11189848A - 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 under the environment where high-concentration sulfuric acid is coagulated. SOLUTION: This steel has a composition consisting of, by weight, <=0.05% C, 0.05-1.0% Si, 0.1-2.0% Mn, 12-27% Ni, 16-26% Cr, >3.0-8.0% Cu, 0.5-5.0% Mo, <=0.5% Al, <=0.3% N, <=0.04% P, <=0.005% S, either or both of 0.001-0.01% Ba and 0.005-1.0% Ag, 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 (alloy) proposed in this publication
Has a low Cu content, so that, 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.

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 tube, 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 weight%, C: 0.05% or less, Si: 0.05-1.0%, Mn: 0.1-2.0.
%, Ni: 12 to 27%, Cr: 16 to 26%, Cu:
More than 3.0% and 8.0% or less, Mo: 0.5 to 5.0
%, Al: 0.5% or less, N: 0.3% or less, P: 0.
04% or less, S: 0.005% or less, and Ba:
0.001 to 0.01% and Ag: 0.005 to 1.0
% Austenitic stainless steel comprising Fe and unavoidable impurities and excellent in sulfuric acid corrosion resistance. "

[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, Cu, which electrochemically suppresses anode active dissolution and suppresses hydrogen generation as a cathode reaction, is contained in an amount exceeding 3% by weight,
Ba and / or have the effect of forming an insoluble sulfate film in sulfuric acid, and this film has an effect of suppressing dissolution on the surface of the steel material.
Alternatively, it has been found that when an appropriate amount of Ag is contained, the corrosion resistance is significantly improved. When Cu, Ba and / or Ag are contained, sufficient corrosion resistance is secured to the austenitic stainless steel to which Cu, Cr and Mo are added in combination, even if N is contained in an amount of 0.05% by weight or more. It became clear that it could be done.

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: C has the effect of increasing the strength,
Since it combines with Cr to form a Cr carbide at a grain boundary and lowers intergranular corrosion resistance, the content is made 0.05% or less. When it is necessary to increase the strength, the content exceeds 0.03% to 0.0
Up to 5% may be contained, but when priority is given to ensuring corrosion resistance, the content of C is desirably 0.03% or less.

Si: Si has a deoxidizing effect. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, when the content exceeds 1.0%, the reduction in hot workability is promoted, and it becomes difficult to form a desired product on an industrial scale together with an increase in the amount of Cu added. Therefore, the content of Si is set to 0.05 to 1.0%.

Mn: 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: Ni has the 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 set to 12 to 27%.

Cr: Cr is an element effective in ensuring the corrosion resistance of austenitic stainless steel. Particularly, in austenitic stainless steel, C of 16% or more
r is the amount of Cu, Mo described below, and further Ba and / or A
When contained together with g, good corrosion resistance can be ensured in an "environment in which high-concentration sulfuric acid condenses". But,
When a large amount of Cr is contained, Cu, Mo, Ba and / or
Alternatively, even in the case of austenitic stainless steel in which Ag is added in a complex manner, the corrosion resistance in the above environment may be rather deteriorated. 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, when the content of Cr is 16
To 26%. From the viewpoint of hot workability, the content of Cr is preferably set to 23% or less.

Cu: Cu is an essential element for ensuring corrosion resistance in a sulfuric acid environment. By containing more than 3.0% of Cu together with the above-described amount of Cr and the below-described amount of Mo, and further with Ba and / or Ag, it is favorable for austenitic stainless steel in an "environment in which high-concentration sulfuric acid solidifies". High corrosion resistance can be imparted. Cr, M
o Further, since the effect of improving corrosion resistance is greater as the content of Cu added in combination with Ba and / or Ag is greater, the Cu content is preferably at least 4.0%. A more preferred content of Cu is 5.0% or more. Incidentally, by increasing the content of Cu, the corrosion resistance in the environment is improved, but the hot workability is reduced, and particularly, the content of Cu is 8.0.
%, Significant deterioration in hot workability occurs. Therefore, the content of Cu is set to more than 3.0% and not more than 8.0%.

Mo: Mo is an element effective for ensuring the corrosion resistance of austenitic stainless steel. Especially,
When 0.5% or more of Mo is contained together with the above-described amounts of Cu and Cr and the amounts of Ba and / or Ag described below,
In the “environment where high concentration sulfuric acid condenses” already mentioned,
Good corrosion resistance can be imparted to austenitic stainless steel. However, when a large amount of Mo is contained, the hot workability is reduced. In particular, when the content of Mo exceeds 5.0%, the hot workability is significantly deteriorated. Therefore, the content of Mo is set to 0.5 to 5.0%. Note that M
The content of o is preferably set to 1.0 to 5.0%.

Al: If the content of Al exceeds 0.5%, the hot workability of the austenitic stainless steel decreases. 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: If the N content exceeds 0.3%, the hot workability of the austenitic stainless steel decreases. Therefore, the content of N is set to 0.3% or less.
In order to ensure extremely good hot workability, N
Is preferably less than 0.05%.

P: Since P deteriorates hot workability and corrosion resistance, the lower the content of P, the better, especially 0.04%
If it exceeds 300, the corrosion resistance in the "environment where high-concentration sulfuric acid condenses" is significantly deteriorated. Therefore, the content of P is set to 0.0
4% or less.

S: S is an element that deteriorates hot workability, and if its content exceeds 0.005%, the hot workability is significantly reduced. Therefore, the content of S is set to 0.005% or less. The S content is 0.003
% Is preferable.

Ba, Ag: When Ba and / or Ag are contained together with the above-mentioned amounts of Cr, Cu and Mo, the corrosion resistance of the austenitic stainless steel in the above-mentioned "environment in which high-concentration sulfuric acid solidifies" is improved. Has an action. However, the contents of Ba and Ag are each 0.001.
% Or less than 0.005%, the desired effect cannot be obtained. On the other hand, Ba and Ag are respectively Ba: 0.01%, A
g: Even if the content exceeds 1.0%, the above effect is saturated and the cost is increased. Therefore, Ba: 0.
One or more of 001 to 0.01% and Ag: 0.005 to 1.0% are contained.

[0036]

EXAMPLES Austenitic stainless steel having the chemical composition shown in Table 1 was melted using a 17 kg high-frequency vacuum melting furnace. Steels 1 to 10 in Table 1 are steels of the examples of the present invention, and steels 11 to 14 are steels of comparative examples in which any of the components is out of the range of the content specified in the present invention.

[0037]

[Table 1]

Next, the ingots of these steels were subjected to hot forging, hot rolling, and solution heat treatment in the usual manner to give a thickness of 6 mm ×
A plate having a width of 100 mm and a length of 700 mm was manufactured. A corrosion test piece having a thickness of 3 mm, a width of 10 mm, and a length of 40 mm was prepared from the solution-heat-treated plate material thus obtained by machining, the surface was wet-polished with # 600 emery paper, degreased with acetone, and sulfuric acid was removed. It was subjected to a corrosion test.

As a test for simulating an environment in which sulfuric acid forms dew, a sulfuric acid spray test was performed. That is, dry air was used as a spray gas, and sulfuric acid having a concentration of 70% was sprayed onto the corrosion test piece at a temperature of 100 ° C. at a deposition rate of 15 mg / (cm ² · h). The spraying time was 5 hours, and the change in weight before and after the test was measured to calculate the corrosion rate.

Table 2 shows the results of the sulfuric acid spray test.

[0041]

[Table 2]

From Table 2, it can be seen that in the steels 1 to 3 and steel 10 of the present invention containing specified amounts of Cr, Cu, Mo and Ag,
The corrosion rate was 0.40 to 0.71 g / (m ² · h), indicating that the sulfuric acid corrosion resistance was excellent.

The corrosion rates of the steels 4 to 6 of the present invention containing specified amounts of Cr, Cu, Mo and Ba are also from 0.21 to 0.
It is understood that the sulfuric acid corrosion resistance is excellent at 80 g / (m ² · h).

Further, a prescribed amount of Cr, Cu, Mo and Ba
Also, the steels 7 to 9 of the present invention containing Ag and Ag have a corrosion rate of 0.55 to 0.71 g / (m ² · h), and it is clear that the sulfuric acid corrosion resistance is excellent.

In the case of steel 10, a specified amount of Cr,
Since Cu, Mo and Ag are contained in combination, sufficient sulfuric acid corrosion resistance is obtained even if the N content exceeds 0.05%.

On the other hand, a prescribed amount of Cr, Cu, M
The corrosion rate of the steel 11 of Comparative Example, which contains o but does not contain any of Ba and Ag, is 2.8 g / (m ² · h) and is inferior to sulfuric acid corrosion resistance in comparison with the steel of the present invention. I have.

Further, as in the case of the steel 14 of the comparative example, even if it contains a prescribed amount of Cr, Cu, and Mo, it does not contain any of Ba and Ag, and the N content is 0.05%. In the case where it exceeds 3, the corrosion rate is 3.5 g / (m 2 · h), and it is clear that the sulfuric acid corrosion resistance is inferior.

The steel 1 of the comparative example which does not contain any of Cu and Mo and further does not contain any of Ba and Ag
2 and steel 13, the corrosion rate was 52.5 g / (m ²
h) and 29.3 g / (m ² · h), it is clear that sulfuric acid corrosion resistance is extremely poor.

[0049]

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 components such as chimneys and chimneys.

Claims (1)

[Claims] (1) In weight%, C: 0.05% or less, Si:
0.05-1.0%, Mn: 0.1-2.0%, Ni:
12 to 27%, Cr: 16 to 26%, Cu: more than 3.0% to 8.0% or less, Mo: 0.5 to 5.0%, Al:
0.5% or less, N: 0.3% or less, P: 0.04% or less, S: 0.005% or less, and Ba: 0.001
Austenitic stainless steel containing at least one of 0.01% and 0.01% of Ag: 0.005% to 1.0%, with the balance being Fe and unavoidable impurities and having excellent sulfuric acid corrosion resistance.