JPH10237601A - Neutral chloride corrosion resistant austenitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel having excellent crevice corrosion resistance and stress corrosion cracking resistance in a neutral chloride environment and furthermore excellent in sulfuric acid dew point corrosion- resistance and hot workability. SOLUTION: This stainless steel has a compsn. contg., by weight, <=0.05% C, <=1% Si, <=2% Mn, <=0.03% P, <=0.01% S, 18 to 24% Cr, 16 to 26% Ni, 4 to 6.5% Mo, 0.1 to 0.3% N, 0 to 3.0% Cu, 0 to 3.0% W, 0 to 0.05% Al, 0 to 0.005% Ca, 0 to 0.005% Mg and 0 to 0.002% B, in which PI value and Ni-bal. value shown by the formulae I and II satisfy 35<=PI<=50 and -2.5<=Ni-bal.<=1, and the balance Fe with inevitable impurities. The formula I is PI=Cr+3.3(Mo+0.5W)+16N and the formula II is Ni-bal.=Ni+30(C+N)+0.5Mn-1.1(Cr+1.5Si+Mo), where the elemental symbols in each formula denote the weight% of each element in the steel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel having excellent resistance to neutral chloride corrosion, and is particularly suitable as a member for a desulfurization apparatus, a stack and a chimney of a coal-fired boiler used in a thermal power plant. Austenitic stainless steel.

[0002]

Desulfurizer-fired boiler coal or heavy oil BACKGROUND ART thermal power plant, the flue and chimney like, it is known that sulfuric acid dew point corrosion due to SO _X contained in the exhaust gas is generated. Sulfuric acid dew point corrosion means that the exhaust gas contacts the surface of the equipment and the temperature drops, the moisture in the exhaust gas reaches the dew point and condenses on the inner surface of the equipment, and it is combined with SO _X in the exhaust gas, resulting in dense sulfuric acid. This is a phenomenon that corrodes the surface of the surface severely.

As a steel for preventing sulfuric acid dew point corrosion, sulfuric acid dew point is obtained by adding S, Cu, or the like to a low alloy steel containing about 1% by weight (hereinafter, the chemical composition is expressed as% by weight) of Cr. Corroded steel has been put to practical use. However, since sulfuric acid dew point corrosion resistant steel is a low alloy steel, corrosion products are generated on the surface of the steel material. This corrosion product scatters outside from a chimney of a boiler or the like to cause environmental problems.

[0004] As another steel which prevents sulfuric acid dew point corrosion, Japanese Patent Application Laid-Open No. Hei 4-34638 discloses that sulfur is prevented from sulfuric acid dew point corrosion by containing more than 2% and less than 5% Cu.
There is disclosed an austenitic stainless steel in which deterioration of hot workability due to the addition of u is prevented by adding B and lowering the oxygen content. Japanese Patent Publication No. 6-8485 discloses an austenitic stainless steel that is excellent in sulfuric acid dew point corrosion resistance, general corrosion resistance, and crevice corrosion resistance by defining the general corrosion resistance index and the crevice corrosion resistance.

On the other hand, in a desulfurization unit, a flue, a chimney and the like in a coal-fired boiler of a thermal power plant using coal as a fuel, in addition to the sulfuric acid dew-point corrosion described above, a gap in a high-concentration neutral chloride environment is required. Corrosion and stress corrosion cracking become problems. The fuel for coal-fired boilers is mainly imported coal from overseas. Imported coal is sprayed with large amounts of seawater to prevent spontaneous ignition during storage and transportation.
Burning such imported coal increases the chlorine concentration in the exhaust gas. This chlorine dissolves and concentrates in the lime slurry generated in the flue gas desulfurization unit. Therefore, the members of the flue and the chimney of the flue gas desulfurization device are exposed to a neutral chloride aqueous solution environment mainly composed of CaCl ₂ and MgCl ₂ .

[0006] In a neutral chloride aqueous solution environment, crevice corrosion and stress corrosion cracking are liable to occur in a flue of a flue gas desulfurization unit or a connection portion between members of a chimney. Crevice corrosion occurs when the dissolved oxygen in the gap decreases. If the dissolved oxygen decreases in the gap, the passivation film of stainless steel is locally destroyed, and Fe ions are eluted. In a neutral chloride aqueous solution environment, Cl ions are present in gaps, so that Fe ions are further eluted to maintain electrical neutrality, and corrosion proceeds. On the other hand, since the stress corrosion cracking occurs from the cutout portion of the material, the portion where the crevice corrosion has progressed is a portion where stress corrosion cracking is likely to occur at the same time.

The austenitic stainless steels described in the above-mentioned Japanese Patent Application Laid-Open No. 4-34638 and Japanese Patent Publication No. 6-8485 are steels developed for the purpose of improving sulfuric acid dew point corrosion resistance. It is not possible to prevent crevice corrosion and stress corrosion cracking occurring in members used in a neutral chloride environment such as a coal-fired boiler.

[0008]

An object of the present invention is to provide excellent crevice corrosion resistance and stress corrosion cracking resistance in a neutral chloride environment, as well as sulfuric acid dew point corrosion resistance and hot workability. It is to provide an austenitic stainless steel.

[0009]

The gist of the present invention resides in the following austenitic stainless steel.

[In weight%, C: 0.05% or less, Si:
1% or less, Mn: 2% or less, P: 0.03% or less, S:
0.01% or less, Cr: 18 to 24%, Ni: 16 to 2
6%, Mo: 4 to 6.5%, N: 0.1 to 0.3%, C
u: 0 to 3%, W: 0 to 3%, Al: 0 to 0.05%,
Ca: 0 to 0.005%, Mg: 0 to 0.005%,
B: 0-0.002% and the following (1),
(2) When the PI value and the Ni-bal. Value represented by the equation are 35 ≦ PI ≦
A neutral chloride-resistant austenitic stainless steel which satisfies 50, -2.5≤Ni-bal.≤1, and the balance is Fe and unavoidable impurities.

PI = Cr + 3.3 (Mo + 0.5W) + 16N (1) Ni-bal. = Ni + 30 (C + N) + 0.5Mn-1.1 (Cr + 1.5Si + Mo) ... (2) Here, the element symbols in the above formulas (1) and (2) represent the weight percentage of each element in the steel. In the austenitic stainless steel of the present invention (hereinafter also referred to as the present invention steel), the following technical means improve crevice corrosion resistance, stress corrosion cracking resistance, sulfuric acid dew point corrosion resistance, and hot workability. ing.

(A) Crevice corrosion resistance Since the steel of the present invention stipulates the PI value of the above formula (1) to be 35 or more, it is used for a member in contact with a neutral chloride aqueous solution such as a coal-fired boiler. However, sufficient crevice corrosion resistance can be maintained. This is because the surface of the steel of the present invention continues to be passivated even at a connection portion between members, that is, a gap portion such as a flange portion or a welded portion. The PI value of the steel of the present invention means Cr, Mo, N and W which improve the corrosion resistance of stainless steel.
Is expressed as Cr equivalent.

(B) Resistance to Stress Corrosion Cracking A member of a coal fired boiler receives a stress of several tens of N / mm ² . Even under such stress loading conditions, the steel of the present invention regulates the PI value of the above formula (1) to 35 or more and sets the Ni content to 16% or more, so that the coal-fired boiler or the like can be used. Sufficient stress corrosion cracking resistance as a member can be maintained. When the PI value is specified to be 35 or more, crevice corrosion resistance is improved as described in the above (A). Since stress corrosion cracking often occurs starting from a notched portion due to crevice corrosion, preventing crevice corrosion in advance also prevents stress corrosion cracking.

Therefore, the steel of the present invention has a PI value of 35 or more in order to improve the stress corrosion cracking resistance. In addition, excellent stress corrosion cracking resistance is obtained by also defining the amount of Ni having a large effect of suppressing stress corrosion cracking in a neutral chloride environment.

(C) Sulfuric acid dew point corrosion resistance The steel of the present invention has a higher content of Cr, Ni and Mo in steel than SUS304 or SUS316 steel. For this reason, it has excellent corrosion resistance in a sulfuric acid immersion environment, and has sufficient sulfuric acid dew point corrosion resistance as a member for a boiler for burning coal only. Further, by including Cu in the steel, sulfuric acid dew point corrosion resistance is further improved.

(D) Hot workability Stainless steel having a high content of Cr, Ni and Mo is:
Cracks are likely to occur during hot forging or hot rolling. However, in the steel of the present invention, the Ni-bal. Value defined by the equation (2) is -0.1.
By defining the range from 25 to 1, the hot workability is improved. The Ni-bal. Value in equation (2) is
Ni, C, N and M which are austenite forming elements
From the sum of the n content multiplied by each constant, δ-
It is a value obtained by subtracting the sum of the contents of Cr, Si, and Mo, which are the elements forming ferrite, and their respective constants, and is an index that indicates the ease with which austenite is formed. When the Ni-bal. Value is in the range of -0.25 to 1, several percent of δ-ferrite and the remainder are austenite phases, and the hot workability is good.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Crevice corrosion resistance in a neutral salt environment,
The effect of each alloy element of the steel of the present invention on the stress corrosion cracking resistance, sulfuric acid dew point corrosion resistance and hot workability and the content thereof will be described.

C: 0.05% or less C deteriorates crevice corrosion resistance and stress corrosion cracking resistance, so it was made 0.05% or less. On the other hand, C is preferably contained in an amount of 0.01% or more from the viewpoint of securing the strength necessary for the use of the steel of the present invention and facilitating melting on a commercial scale.

Si: 1% or less Si, when excessively contained, deteriorates hot workability, toughness and weldability of steel, so the upper limit of the content is set to 1%.
On the other hand, Si has a deoxidizing effect, and in order to exhibit its effect, it is desirable to contain 0.1% or more.

Mn: 2% or less Mn is an element for stabilizing austenite, and is desirably added to reduce the amount of expensive Ni having the same effect. However, if the content is excessive, the crevice corrosion resistance and stress corrosion cracking resistance of the steel are deteriorated. Therefore, the upper limit of the content is set to 2%. On the other hand, Mn has a deoxidizing effect similarly to Si, and in order to exhibit its effect, it is 0.1%.
% Is desirably contained.

P: not more than 0.03% P is one of the unavoidable impurities that are difficult to completely remove during the production process of steel, and has resistance to crevice corrosion, stress corrosion cracking and hot workability. Worsen. Therefore, the content is set to 0.03% or less.

S: 0.01% or less S is one of the unavoidable impurities that are difficult to completely remove during the steel manufacturing process. If a large amount of S is present in steel, Mn
And MnS. MnS significantly deteriorates crevice corrosion resistance. Therefore, the content of S is 0.01
% Or less.

Cr: 18 to 24% Cr is one of the elements that improve crevice corrosion resistance, stress corrosion cracking resistance and sulfuric acid dew point corrosion resistance in a neutral chloride environment. In order to maintain high crevice corrosion resistance even in a neutral chloride environment, even if Ni, Mo and N coexist, C
r must be contained at least 18%. The more Cr is added, the more the corrosion resistance is improved, but if it exceeds 24%, a large amount of δ-ferrite ferrite phase is formed, and the hot workability is deteriorated. Therefore, the content of Cr is set to 18 to 24%.

Ni: 16-26% Ni is an element that stabilizes austenite. Ni improves stress corrosion cracking resistance and sulfuric acid dew point corrosion resistance in a neutral chloride environment. In order to maintain the required stress corrosion cracking resistance in a neutral chloride environment, Ni
Must be contained at least 16%. However, if the content exceeds 26%, the effect saturates and only the manufacturing cost increases, so the upper limit is set to 26%.

Mo: 4 to 6.5% Mo is one of the elements for improving crevice corrosion resistance and stress corrosion cracking resistance in a neutral chloride environment. In order to maintain the required crevice corrosion resistance and stress corrosion cracking resistance required in a neutral chloride environment, which is an environment in which the steel of the present invention is used, Mo must be set to 4%.
% Or more. However, if the content exceeds 6.5%, the effect is saturated and only the production cost increases, so the upper limit is set to 6.5%.

N: 0.1 to 0.3% N improves the corrosion resistance by coexisting with Cr, Ni and Mo. In particular, it improves crevice corrosion resistance in a neutral chloride environment. The effect is exhibited by containing 0.1% or more. However, when the content exceeds 0.3%, not only the effect is saturated, but also nitrides are formed at the grain boundaries of stainless steel, which deteriorates crevice corrosion resistance, stress corrosion cracking resistance, and hot workability. , The upper limit of the content is 0.1.
3%.

Cu: 0 to 3% Although Cu may not be added, Cu improves sulfuric acid dew point corrosion resistance, crevice corrosion resistance and stress corrosion cracking resistance.
It is desirable to add. In order to exert its effect sufficiently, it is preferable to contain 0.5% or more. However, when the content exceeds 3%, hot workability and weldability are deteriorated.
Therefore, the upper limit was set to 3%.

W: 0 to 3% Although the addition of W is not essential, it is desirable to add W because it improves crevice corrosion resistance and stress corrosion cracking resistance. In order to exert its effect sufficiently, it is preferable to contain 0.5% or more. However, if the content exceeds 3%, the effect is saturated and only the production cost is increased. Therefore, the upper limit was set to 3%.

Al: 0 to 0.05% Al has a deoxidizing effect and is added as necessary. In order to exert its effect sufficiently, it is preferable to contain 0.005% or more. However, if the content exceeds 0.05%, not only does the deoxidizing effect become saturated, but also the weldability deteriorates, so the upper limit was made 0.05%.

Ca: 0 to 0.005% Ca is an element added as necessary as a deoxidizing or desulfurizing element. It also has the effect of improving hot workability. In order to sufficiently exhibit these effects, 0.0002% or more is preferably contained. However, when the content exceeds 0.005%, the effect is saturated. Therefore, the upper limit of the content is 0.00.
5%.

Mg: 0 to 0.005% Mg is an element added as necessary as a deoxidizing or desulfurizing element. It also has the effect of improving hot workability. In order to sufficiently exhibit these effects, 0.0002% or more is preferably contained. However, when the content exceeds 0.005%, the effect is saturated. Therefore, the upper limit of the content is 0.00.
5%.

B: 0 to 0.002% B is an element to be added as necessary, because it improves hot workability. To fully demonstrate its effect, 0.0
The content is preferably 002% or more. However, when the content exceeds 0.002%, the precipitation of carbides at the grain boundary is promoted, and the crevice corrosion resistance and stress corrosion cracking resistance are deteriorated. Therefore, the upper limit of the B content is set to 0.002%.

PI value: 35 ≦ PI ≦ 50 In order to maintain the required crevice corrosion resistance and stress corrosion cracking resistance in a neutral chloride environment, the content of each element of the steel of the present invention must be within the above specified range. In addition to the above, it is necessary to adjust the content of each element so that the PI value represented by the above formula (1) is 35 or more and 50 or less. If the PI value is less than 35, crevice corrosion resistance and stress corrosion cracking resistance are insufficient. on the other hand,
When the PI value exceeds 50, not only does the raw material cost rise, but also the hot workability deteriorates. Therefore, P
The upper limit of the I value was set to 50.

Ni-bal. Value: -2.5 ≦ Ni-bal. ≦ 1 In order to ensure good hot workability, the content of each element of the steel of the present invention is set within the above specified range. In addition, the above (2)
It is necessary to adjust the content of each element so that the Ni-bal. Value shown by the equation is -0.25 or more and 1 or less. Ni-ba
If the l. value is less than -0.25 or more than 1, cracks occur during hot forging or hot rolling.

[0035]

EXAMPLES After the steel of the present invention was melted and formed into a thick plate, its crevice corrosion resistance, stress corrosion cracking resistance, sulfuric acid dew point corrosion resistance and hot workability were investigated.

Table 1 shows the steels of the present invention and the steels of the present invention which were produced by the vacuum melting method and have different compositions.
Abbreviated as comparative steel. ). This forged steel was subjected to hot forging by an ordinary method, and then hot-rolled to form a hot-rolled sheet having a thickness of 6 mm. During hot rolling, cracks generated in the hot-rolled sheet were visually observed. After that, 1
A solution heat treatment was performed at 150 ° C. for 30 minutes.

[0037]

[Table 1]

(A) Crevice corrosion resistance Test pieces of two sizes shown in FIG. 1 were cut out from hot-rolled sheets of the present invention steel and comparative steel, wet-polished to # 600 emery paper, and acetone-degreased. . The gap was reproduced by restraining the test pieces of two sizes with bolts. The bolt was insulated with a Teflon washer. The crevice corrosion test is based on the assumption that actual drying and wetting of flue and chimney of flue gas desulfurization equipment is repeated.
Dry-wet was performed for 144 cycles (720 h). The salt water is added to distilled water at a ratio of 5: 1 CaCl ₂ and MgCl ₂ so as to have a chlorine ion concentration in a lime slurry of a flue gas desulfurization unit of a coal-fired boiler, and sulfuric acid is used. Was adjusted to pH 5 and the chloride ion concentration was adjusted to 20.
000 ppm.

The weight loss after the test was measured, and assuming that the corrosion occurred only in the gap, the weight loss divided by the total area of the gap (hereinafter referred to as corrosion weight loss) was used as the gap resistance. The corrosivity was evaluated. The results are shown in Table 2.

[0040]

[Table 2]

The steel of the present invention has a corrosion loss of 3.2 g / m
² or less, indicating good crevice corrosion resistance. On the other hand, Comparative Steels 15 to 18 and 20 had a corrosion weight loss of 3.7 g / m.
^It exceeds ² and is inferior in crevice corrosion resistance.

Further, the relationship between the PI value of the steel of the present invention and the comparative steel and the weight loss due to corrosion was also investigated. The result is shown in FIG. It was found that when the PI value was 35 or more, the corrosion weight loss was 3.2 g / m ² or less.

(B) Stress corrosion cracking resistance A stress corrosion cracking test was performed by the U-bend test method. As shown in FIG. 3, the test piece was 10 mm wide and 75 mm long from the hot rolled sheet.
mm, wet-polished to # 600 emery paper, degreased with acetone, sandwiched a Teflon sheet between two test pieces, and bent into a U-shape with a punch and roller.
Further, the U-shaped interval was narrowed by 5 mm with a bolt to apply a stress. Stress corrosion cracking in a neutral chloride environment occurred more severely in the gap, so that stress was applied to the test piece and the gap was formed.

The same chloride ion concentration as in (A) above
Salt spray-dry-wet using 1 000 ppm saline
Forty-four cycles (720 h) were performed to investigate the occurrence of stress corrosion cracking. At the same time, the corrosion loss of the gap was also measured. The results are shown in Table 2. No stress corrosion cracking occurred in the steel of the present invention. The corrosion weight loss in the gap was as good as ² g / m ² or less. On the other hand, stress corrosion cracking occurred in Comparative Steels 1, 19 and 20.

(C) Hot workability Cracks generated in the hot rolled sheet during hot rolling were observed. The results are shown in Table 2. No crack occurred in the hot-rolled sheet of the steel of the present invention. On the other hand, the hot-rolled sheets of comparative steels 20 and 21 are Ni-bal.
Was below -2.5, causing cracking. Since the Ni-bal. Of the hot-rolled sheet of Comparative Steel 22 exceeded 1, cracking occurred.

(D) Sulfuric Acid Dew Point Corrosion Resistance Test pieces were cut out from hot-rolled sheets of the steel of the present invention and the comparative steel, immersed in sulfuric acid, and examined for sulfuric acid dew point corrosion resistance. The test piece was cut into a shape of 10 mm in width and 40 mm in length from the hot-rolled sheet, wet-polished to # 600 emery paper, and degreased with acetone. Test pieces of the steel of the present invention and the comparative steel were 80
%, Immersed in a 140 ° C. sulfuric acid aqueous solution for 4 hours, and then measured the corrosion rate. Corrosion rate is corrosion loss per hour [g / m ² ·
h]. The test results are shown in Table 2. The corrosion rates of the steels of the present invention were all less than 100 g / m ² / h, indicating good sulfuric acid dew point corrosion resistance. On the other hand, comparative steels 15 to 18, 2
In Nos. 0 and 22, the corrosion rate exceeds 100 g / m ² / h, and the sulfuric acid dew point corrosion resistance is insufficient.

[0047]

The austenitic stainless steel of the present invention has excellent crevice corrosion resistance and stress corrosion cracking resistance in a neutral chloride environment, and also has sulfuric acid dew point corrosion resistance and hot workability. It is suitable as a member used in a neutral chloride environment such as a member for a coal-fired boiler.

[Brief description of the drawings]

FIG. 1 is a view showing a crevice corrosion test piece.

FIG. 2 is a diagram showing a relationship between a corrosion weight loss after a crevice corrosion test and a PI value.

FIG. 3 is a diagram showing a test piece in a stress corrosion cracking test and a stress applied state.

Claims (1)

[Claims] (1) In weight%, C: 0.05% or less, Si: 1
% Or less, Mn: 2% or less, P: 0.03% or less, S: 0.1% or less.
01% or less, Cr: 18 to 24%, Ni: 16 to 26
%, Mo: 4 to 6.5%, N: 0.1 to 0.3%, Cu:
0-3%, W: 0-3%, Al: 0-0.05%, C
a: 0 to 0.005%, Mg: 0 to 0.005%, B:
0-0.002%, and the PI value and Ni-bal. Value represented by the following formulas (1) and (2) are 35 ≦ PI ≦ 50,
A neutral chloride-corrosive austenitic stainless steel which satisfies -2.5 ≦ Ni-bal. ≦ 1 and the balance consists of Fe and unavoidable impurities. PI = Cr + 3.3 (Mo + 0.5W) + 16N (1) Ni-bal. = Ni + 30 (C + N) + 0.5Mn-1.1 (Cr + 1.5Si + Mo) (2) Here, the element symbols in the above formulas (1) and (2) represent the weight percentage of each element in the steel.