JPH1018004A - Austenitic stainless steel excellent in sulfuric acid corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material having corrosion resistance to sulfuric acid concns. in a wide range. SOLUTION: This austenitic stainless steel is the one having a compsn. contg., by mass, <=0.05% C, <=1.0% Si, <=2.0% Mn, <=0.01% S, <=0.1% Al, 15.0 to 27.0% Ni, 17.0 to 26.0% Cr, 2.0 to 8.0% Cu, 0.1 to 2.0% Mo, and the balance Fe with inevitable impurities or is the one having a compsn. contg., by mass, <=0.05% C, <=1.0% Si, <=2.0% Mn, <=0.01% S, <=0.1% Al, 15.0 to 27.0% Ni, 17.0 to 26.0% Cr, 2.0 to 8.0% Cu and 0.1 to 2.0% Mo, furthermore contg. one or more kinds of elements selected from 0.005 to 0.01% Ca, 0.0005 to 0.01% B and 0.0005 to 0.01% rare earth elements, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel pipe for a heat exchanger used in a boiler for thermal power generation or an industrial boiler, or a material used for an exhaust gas treatment facility provided in a flue, a chimney or a flue. Austenitic stainless steel with excellent sulfuric acid corrosion resistance.

[0002]

2. Description of the Related Art Petroleum and coal used as fuel for thermal power boilers contain sulfur (S). Combustion gas containing sulfur oxides (S0x) generated by combustion causes a decrease in gas temperature and gas. The moisture in the water forms sulfuric acid, which forms dew on the equipment and corrodes materials such as heat exchangers. Therefore, in order to prevent the dew condensation of sulfuric acid, the gas temperature in the heat exchanger was maintained at a temperature (150 ° C.) at which sulfuric acid did not form a dew point. However, in view of the recent increase in energy demand and effective use of energy, in order to recover thermal energy as effectively as possible, for example, the temperature of exhaust gas from a heat exchanger tends to be low, and a corrosion-resistant material for sulfuric acid is required. It became so. On the other hand, in stacks and chimneys, medium-concentration (about 50%) sulfuric acid is condensed at low temperatures (100 ° C or lower), so corrosion-resistant materials are also required.

[0003] In a region (for example, 140 ° C) where the gas temperature of a conventional heat exchanger is lower than 150 ° C, high-concentration sulfuric acid of about 80% is condensed. Alloy steel has been used. The reason for this is that low alloy steel has better corrosion resistance than stainless steel for sulfuric acid at high temperature and high concentration (for example, 100 ° C. or more, 70% or more) as described above. On the other hand, when the temperature further decreases, the sulfuric acid concentration also becomes 50%, and conversely, low alloy steel is more corrosive than stainless steel. For example, the following proposals have been made as sulfuric acid corrosion resistant materials for these specific environments.

[0004] In low alloy steel, (1) Cu: 0.15 to 0.60%, Cr: 5.00% or less, B: 0.01 to 0.
Sulfur dew point corrosion resistant steel containing 50% (Japanese Patent Publication No. 47-18764).

(2) Si: 1.5 to 7.0%, Cr: 2% or less, C
u: 0.2-0.5%, S: 0.03% or less, Mn: 0.2% or less,
P: excellent Si-containing steel in a sulfuric acid corrosion environment containing 0.02% or less and a sulfuric acid concentration of less than 30% (JP-A-61-3867)
No.).

[0006] In stainless steel, (1) Cr: 18.0-29.0%, Ni: 20.0-45%, Mo: 4.0-9.
0%, Si: 1.5 to 5.0%, Cu: 0.5 to 3.0%, Mn: 2.0
%, C: 0.10% or less, at a temperature of around 100 ° C,
Sulfuric acid corrosion resistant stainless steel with a concentration of 95% or more (Japanese Patent Application Laid-Open No. 56-93860).

(2) Cr: 18 to 25%, Ni: 14 to 24%, Mo: 1
4.5%, Si: 5% or less, Cu: 0.5-2.0%, Mn: 2.0
% Or less, C: 0.004 to 0.05%, N: 0.01 to 0.3%, the two characteristics of crevice corrosion resistance and general corrosion resistance by regulating the overall corrosion resistance index and the crevice corrosion resistance index. High alloy stainless steel for chimneys, flue and desulfurization equipment satisfying (JP-A-2-170946).

(3) Cr: 15 to 28%, Ni: 8.0 to 30%, Mo:
2% to 7% or less, Si: 1.00% or less, Cu: 2% to 5%
Hereinafter, Mn: 2.0% or less, C: 0.050 or less, N: 0.05-0.
Contains 35%, low oxygen, contains B, and Cu, M
Stainless steel for chimney stack with excellent hot workability and sulfuric acid corrosion resistance by specifying the contents of o, B and oxygen
(JP-A-4-346638).

[0009]

As described above, in order to increase the energy recovery efficiency of the heat exchanger, it is necessary to further lower the gas temperature. However, when the temperature becomes low (about 70 ° C.), the sulfuric acid concentration also becomes 50%. The corrosion of low alloy steel is greater than that of stainless steel. For this reason, the low alloy sulfuric acid dew-point corrosion resistant steel conventionally used cannot be used in a part where the gas temperature continuously changes from a high temperature to a low temperature. For the same reason, stainless steel cannot be used due to poor corrosion resistance on the high temperature side. Therefore, there has been a demand for a material that exhibits corrosion resistance in a wide range of sulfuric acid solutions from a high temperature of about 140 ° C. to a low temperature of about 70 ° C. However, as mentioned above, although there is a proposal for a sulfuric acid corrosion resistant material in a specific environment, the concentration is high (80%) to medium to low (70-30%).
%), No material satisfies the resistance to sulfuric acid corrosion in all environments up to (%).

[0010] An object of the present invention is to provide a material having corrosion resistance to sulfuric acid at a high temperature and a high concentration to a medium and low concentration at which the sulfuric acid condenses. Hereinafter, low temperature is defined as “100 ° C
"Lower", "high temperature" means "100 ° C or more", "high concentration" means "70% or more", and "medium / low concentration" means "less than 70% to 30%". Further, the sulfuric acid having a high temperature and a high concentration to a medium and low concentration may be simply referred to as “a wide range of sulfuric acid concentration”.

[0011]

The present inventors conducted a corrosion resistance test on a wide range of sulfuric acid concentrations using austenitic stainless steel, and as a result, contained a large amount of Cu,
It was found that by limiting the amount of Mo, the corrosion resistance to sulfuric acid from high to medium and low concentrations was improved.

The present invention has been completed based on the above findings,
The gist lies in the austenitic stainless steel shown below (hereinafter,% relating to the content of alloying elements means mass%).

C: 0.05% or less, Si: 1.0% or less, M
n: 2.0% or less, S: 0.01% or less, Al: 0.10% or less, N
i: 15.0-27.0%, Cr: 17.0-26.0%, Cu: 2.0-8.0
%, Mo: 0.1-2.0%, austenitic stainless steel with excellent resistance to sulfuric acid corrosion composed of balance Fe and unavoidable impurities.

C: 0.05% or less, Si: 1.0% or less, Mn:
2.0% or less, S: 0.01% or less, Al: 0.10% or less, Ni: 1
5.0-27.0%, Cr: 17.0-26.0%, Cu: 2.0-8.0%, M
o: 0.1 to 2.0%, and one or more elements selected from Ca, B, and rare earth elements are 0.0005 to 0.01
%, B is 0.0005 to 0.01%, rare earth element is 0.0005 to 0.01%
Austenitic stainless steel with excellent resistance to sulfuric acid corrosion composed of Fe and inevitable impurities.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention contains a large amount of Cu,
Ni-Cr austenitic stainless steel with a limited amount of Mo. Hereinafter, the reason for defining the chemical composition will be described.

C: C enhances the strength of the steel material, but is a Cr carbide forming element at the grain boundary and lowers the intergranular corrosion resistance, so that it is suppressed to 0.05% or less. From the point of the intergranular corrosion resistance, it is better to be as small as possible. But to increase the strength
You may make it contain in the range of 0.03-0.05%.

Si: Si is required as a deoxidizing element.
It can be contained up to 0%. However, in the case of high Cu content steel, the amount of solid solution of Cu is reduced and hot workability is deteriorated.
It is desirable to make it up to 0.5%.

Mn: Mn is an element that lowers the corrosion resistance, but it fixes S and improves the hot workability and stabilizes the austenite phase, so that up to 2.0% is acceptable. The lower limit may be such that it is inevitably mixed.

S: S is an element that deteriorates the corrosion resistance and hot workability of steel, and its content is preferably as small as possible. 0.01% is the allowable upper limit.

Al: If the Al content exceeds 0.10%, the hot workability of the steel decreases. Therefore, the upper limit is 0.10%. The lower limit may be the range of unavoidable impurities. However, Al
Is effective as a deoxidizing agent for steel, so when the above-mentioned Si is kept low, it may be positively added to supplement its deoxidizing action. In order to sufficiently perform deoxidation, it is desirable to contain 0.01% or more.

Ni: Ni is an element that improves the corrosion resistance of steel and stabilizes the austenite phase. To ensure these effects, a content of 15.0% or more is required. When the Ni content exceeds 18.0%, the corrosion resistance in a sulfuric acid solution is remarkably improved. However, if it exceeds 27.0%, the effect will be saturated and the cost will only increase. Therefore, the content is set to 15.0 to 27.0%. A more preferable range of the content is 18.0 to 27.0%.

Cr: Like Ni, Cr improves the corrosion resistance of steel in a sulfuric acid solution, but its effect cannot be obtained at less than 17.0%. In general, Cr is considered to be an element that increases the corrosion resistance of steel as its content increases. However, if it is contained in a sulfuric acid solution in a large amount, the corrosion resistance decreases. Therefore, the content is set to 17.0 to 26.0%. Note that the preferable content is 17.0 to 23.0%.

Cu: Cu electrochemically suppresses anodic active dissolution and also functions to suppress the generation of hydrogen, which is a cathodic reaction, and improves corrosion resistance in a wide range of sulfuric acid concentration solutions. However, if the content is less than 2.0%, there is no effect, and if it exceeds 8.0%, the hot workability of the steel is impaired.
Therefore, its content should be 2.0-8.0%. further,
It is desirable to be 3.0-6.0%.

Mo: Mo was conventionally thought to improve corrosion resistance in a sulfuric acid solution, but was found to deteriorate it in a high-concentration sulfuric acid solution. Therefore, in the present invention, the upper limit of the content of Mo is suppressed to 2.0%. On the other hand, it is effective to contain 0.1% or more of Mo for the corrosion resistance to a sulfuric acid solution having a medium concentration. Therefore, in the steel of the present invention, the content of Mo is set to 0.1 to 2.0%. More preferably 0.5 to
1.5%.

One of the austenitic stainless steels according to the present invention comprises, in addition to the above elements, the balance consisting of Fe and unavoidable impurities. Another steel of the present invention further improves hot workability by adding one or more elements selected from Ca, B and rare earth elements to this steel.

Ca: Ca is added as necessary in order to combine with S to suppress a reduction in hot workability of S. But 0.00
If the content is less than 05%, the effect is not obtained. If the content exceeds 0.01%, the cleanliness of the steel is reduced, which causes flaws during production, which is not preferable. Therefore, its content is made 0.0005 to 0.01%. More preferably, it is 0.001 to 0.01%.

B: B is also an element that improves the hot workability of steel, and is added as necessary. However, if less than 0.0005%, the effect cannot be obtained. In order to exert the effect sufficiently, it is desirable to contain 0.001% or more. But 0.
If it exceeds 01%, the precipitation of Cr-B compounds at the grain boundaries of steel is promoted, and the corrosion resistance is reduced.

Rare earth elements: Rare earth elements such as La and Ce
It is an element that improves hot workability, and is added as necessary. If the total content is less than 0.0005%, the effect of improving hot workability cannot be obtained. On the other hand, if the content exceeds 0.010%, the productivity and pitting corrosion resistance are reduced due to a decrease in cleanliness, which is not preferable. Therefore, the content of rare earth elements in total
0.0005 to 0.01%.

[0029]

EXAMPLES Ingots obtained by smelting steel having the chemical components shown in Table 1 in a 20 kg high-frequency vacuum melting furnace were hot forged to a thickness of 20 mm.
The plate was 100 mm wide and 250 mm long. Furthermore, after hot-rolling this plate material to a thickness of 8 mm, 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 sulfuric acid corrosion test.

[0030]

[Table 1]

The hot workability was determined by, after forging the ingot, observing the state of occurrence of cracks in the obtained sheet material, measuring the maximum crack length, and setting the target value of hot workability to 15 mm. did.

The sulfuric acid corrosion test assumes a dew point corrosion in a heat exchanger, and is performed in a solution having a temperature of 140 ° C. and a sulfuric acid concentration of 80%.
The test was performed in two environments in a solution having a temperature of 70 ° C. and a sulfuric acid concentration of 50%.

In a part where a high concentration of sulfuric acid is condensed in a real environment, the amount is small. Therefore, in a test of a solution having a temperature of 140 ° C. and a sulfuric acid concentration of 80% (hereinafter referred to as “80% H ₂ SO ₄ solution”), immerse the sample in the solution for 5 minutes, pull it out of the solution, and use the solution. The test in which the sample was kept in a wet state at a temperature of 140 ° C. for 55 minutes and then immersed in the solution again was repeated five times. A solution having a temperature of 70 ° C. and a sulfuric acid concentration of 50% (hereinafter referred to as “50% H ₂ SO ₄ solution”) was subjected to continuous immersion for 5 hours. Evaluation of corrosion resistance measures corrosion weight loss,
The corrosion rate per unit area and unit time was g / (m ² · hr). Note that the target value of the corrosion resistance was 4.0 g / (m ² · hr) or less in each case.

Table 1 shows the test results.

Since the steels of the invention examples (steel symbols 1 to 14) contain Cu, Mo and Si within the range defined by the present invention, 80% H
₂ Corrosion resistance in SO ₄ solution is less than 3.21 g / (m ²
The corrosion resistance in a% H ₂ SO ₄ solution is good at 3.86 g / (m ² · hr) or less. The crack length after slab forging is
In each case, the hot workability is good at 15 mm or less. Symbol 4 ~
Steel No. 12 contains 0.25% or less of Si, 0.002% or less of S, and 0.2% of Al.
071% or less and contains at least one of Ca, B, La and Ce, so that the crack length after slab forging is 5
mm or less, and the hot workability is good.

On the other hand, the steel of Comparative Example No. 15, which does not contain Cu, has a corrosion resistance in an 80% H ₂ SO ₄ solution of 42.1.
g / (m ² · hr), corrosion resistance in 50% H ₂ SO ₄ solution is 33.6
Bad with g / (m ² · hr). In addition, the steel with reference numeral 16
Since it contains only 1.5%, the corrosion resistance in an 80% H ₂ SO ₄ solution is 18.4 g / (m ² · hr), and the corrosion resistance in a 50% H ₂ SO ₄ solution is 8.7 g / (m ² · hr) and bad. Further, the steel with reference numeral 17
The hot workability was poor due to the Cu content of 9.2%, and a corrosion test specimen could not be prepared.

Since the steel with reference numeral 18 contains 2.84% of Mo,
The corrosion resistance in an 80% H ₂ SO ₄ solution is as poor as 5.52 g / (m ² · hr). That is, the sulfuric acid dew point corrosion at a relatively high temperature is inferior. Further, since the steel with reference numeral 19 does not contain Mo, its corrosion resistance in a 50% H ₂ SO ₄ solution is poor at 10.1 g / (m ² · hr). That is,
Poor sulfuric acid dew point corrosion at relatively low temperatures.

Since the steel with reference numeral 20 contains only 12.2% of Ni, its corrosion resistance in an 80% H ₂ SO ₄ solution is 14.2 g / (m ² ···
hr), the corrosion resistance in a 50% H ₂ SO ₄ solution is 15.4 g / (m ²
hr) and bad. In addition, since the steel with reference numeral 21 contains only 14.8% of Cr, its corrosion resistance in an 80% H ₂ SO ₄ solution is
6.73 g / (m ² · hr), corrosion resistance in 50% H ₂ SO ₄ solution is 1
Bad with 0.6 g / (m ² · hr). Further, since the steel with reference numeral 22 contains 30.4% of Cr, its corrosion resistance in an 80% H ₂ SO ₄ solution is poor at 5.37 g / (m ² · hr). That is, the sulfuric acid dew point corrosion at a relatively high temperature is inferior.

In the steel of reference numeral 23, S is as high as 0.015%,
The corrosion test could not be performed because the hot workability was poor and a corrosion test piece could not be prepared.

FIG. 1 is a graph showing the relationship between the Cu content of the inventive steel and the weight loss in corrosion in an 80% H ₂ SO ₄ solution. As shown in the figure, the corrosion rate can be significantly reduced by containing 2% or more of Cu. Almost the same tendency is obtained in a 50% H ₂ SO ₄ solution.

FIG. 2 is a graph showing the relationship between the Mo content of the steel used in the test and the corrosion rate in a sulfuric acid solution. From the steels shown in Table 1, almost 5% Cu and almost 20% Cr were used. Almost 25% Ni
It is the figure which put together the relationship between Mo content and corrosion rate about the steel (steel code 4, 7, 8, 9, 13, 14, 18, and 19) containing. From the figure, it can be seen that Mo has different corrosion resistance depending on the concentration of sulfuric acid, and the corrosion rate is higher when Mo is not contained in a 50% H ₂ SO ₄ solution, and when Mo is higher in an 80% H ₂ SO ₄ solution. It turns out that it is big.

Ni is effective for corrosion resistance like Cu.
However, if the Ni content is too low (steel code 20), the corrosion rate increases. Cr has an optimal content for both 50% and 80% sulfuric acid concentrations, and its content is too low (steel sign
21) or too high (steel code 22), the corrosion resistance is reduced.

Regarding the hot workability, steel containing a large amount of S (steel symbols 13 and 14) and steel containing a small amount of Al (steel symbols)
18), compared to steels (steel codes 1 to 3) without addition of Ca or B or La + Ce, steels added with an appropriate amount (steel codes 4 to 12) have fewer cracks and improved workability.

[0044]

The austenitic stainless steel of the present invention has excellent corrosion resistance to a wide range of temperatures and a wide range of concentrations of sulfuric acid solutions, and equipment for condensing flue gas, such as heat exchangers, desulfurization equipment, flues and chimneys. It can be used as a material.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the Cu content of an inventive steel and the weight loss in corrosion in an 80% H ₂ SO ₄ solution.

FIG. 2 is a diagram showing the relationship between the Mo content of steel and the weight loss due to corrosion in a H ₂ SO ₄ solution.

Claims (2)

[Claims] (1) In mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.01% or less, Al: 0.10% or less, Ni: 15.0 to 27.0%, Cr: 17.0 to 26.0%, Cu: 2.0 ~
Austenitic stainless steel with an excellent sulfuric acid corrosion resistance, characterized by 8.0%, Mo: 0.1-2.0%, the balance being Fe and unavoidable impurities. 2. In mass%, C: 0.05% or less, Si: 1.0% or less, Mn: 2.0% or less, S: 0.01% or less, Al: 0.10% or less, Ni: 15.0 to 27.0%, Cr: 17.0 to 26.0%, Cu: 2.0 ~
8.0%, Mo: 0.1 to 2.0%, and one or more elements selected from Ca, B and rare earth elements are 0.0005%
~ 0.01%, B is 0.0005 ~ 0.01%, rare earth element is 0.0005 ~
Austenitic stainless steel excellent in sulfuric acid corrosion resistance and hot workability, characterized in that it is contained in the range of 0.01% and the balance is Fe and unavoidable impurities.