JPH0570895A - High corrosion resistant alloy steel for waste heat incinerator waste heat boiler heat transfer tubes - Google Patents

Abstract

(57) [Abstract] [Purpose] High corrosion resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, which has excellent high temperature strength and stress corrosion cracking resistance, and especially high general corrosion resistance and intergranular corrosion resistance at high temperatures. [Structure] In weight%, C ≦ 0.05%, Si ≦ 2%, Mn ≦
2.5%, 15% ≤ Cr ≤ 30%, 25% ≤ Ni ≤ 50%, 1% ≤
Al ≦ 5%, 0.5% ≦ Mo ≦ [5.8-Ni (%) / 10]
%, High-corrosion resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, the balance of which is Fe and unavoidable impurities. In addition to the above components, at least one of Nb, Ti, Zr and V is added in total.
0.1 to 3% by weight, one or more of Cu, Co and W in total of 0.1 to 5% by weight, one or more of rare earth elements in total of 0.01
.About.0.1% by weight may be added alone or in combination.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention aims to recover energy in a facility that incinerates municipal solid waste and industrial waste, human waste, sludge and the like (hereinafter collectively referred to as "garbage"), and generate power by waste heat. The present invention relates to a highly corrosion-resistant alloy steel having an austenite structure, which is used as a boiler heat transfer tube such as a superheater tube, an evaporation tube, and a water wall tube of a boiler installed for the purpose.

[0002]

2. Description of the Related Art From the viewpoint of positively utilizing unused energy, it has been attempted to effectively utilize the energy of waste. Even at present, some facilities are using the waste heat generated when waste is incinerated to generate electricity to cover district heating and electric power in incineration facilities. However, since garbage is also one of the valuable energy sources left for humankind, various attempts have been made toward power generation, in which the energy originally contained in garbage is positively taken out as electric power energy as much as possible.

When extracting electric energy from garbage,
It is necessary to use the waste heat from waste incineration as effectively as possible, and to improve the power generation efficiency, it is necessary to raise the steam conditions of the waste heat boiler to high temperature and high pressure. For example, in the conventional boiler, the wall temperature of the superheater pipe, which has the highest temperature, is 200-
It is about 350 ℃, but depending on the boiler design, it may be necessary to raise the tube wall temperature to over 500 ℃. However, increasing the temperature of the steam causes the temperature of the wall of the boiler heat transfer tube to increase, and thus the corrosion of the boiler heat transfer tube tends to be intensified. In addition, increasing the pressure of steam requires that the boiler heat transfer tubes have excellent high-temperature strength. Therefore, as a material for the heat transfer tube of the high temperature / high pressure type boiler used for the above purpose, it is necessary to have a material having corrosion resistance capable of withstanding an extremely harsh corrosive environment and having excellent high temperature strength.

In order to develop such a material, the present inventors have taken into consideration the corrosion resistance performance of various corrosion resistant steels and corrosion resistant alloy steels having an austenitic structure excellent in high temperature strength, taking into consideration the corrosive environment of the refuse incinerator in Japan. We conduct a comparative examination under the conditions,
As a result, like a Japanese incinerator, even in a severe corrosive environment where a high concentration of molten chloride adheres to the surface of the pipe, it has excellent general corrosion resistance and exhibits strong resistance to stress corrosion cracking. An austenitic high alloy steel containing Cr and Ni and an austenitic high alloy steel containing Mo in addition to these components were found, and a patent application was previously filed.
(Japanese Patent Application No. 3-48133, filed on March 13, 1991). In the case of steel containing no Mo, the invention of this prior application is based on Cr and Ni.
If the content of is adjusted to an appropriate range, the stress corrosion cracking susceptibility becomes low. In the case of steel containing Mo, the stress corrosion cracking sensitivity depends on Ni and Mo contents, and the Mo content depends on the Ni content. It was based on the finding that the susceptibility to stress corrosion cracking remarkably decreases when adjusted by adjustment.

[0005]

In order to efficiently extract electric energy from waste, it is necessary to raise the steam condition of the waste heat boiler to a higher temperature and pressure, but it is better to set the steam condition to a higher temperature. It is advantageous. The high-alloy steel of the prior invention described above exhibits good high-temperature strength because it has an austenitic structure, and although excellent in general corrosion resistance and stress corrosion cracking resistance, when the steam condition of the waste heat boiler is set to a higher temperature. It is expected that general corrosion is more likely to occur and intergranular corrosion is more likely to occur. An object of the present invention is to provide a highly corrosion-resistant alloy steel for waste incineration waste heat boiler heat transfer tubes, which is less likely to cause wall thickness reduction and intergranular corrosion due to general corrosion even when used at higher temperatures. Specifically, it has an austenitic structure that exhibits excellent strength at high temperatures as well as the high alloy steel of the prior invention, while maintaining resistance to stress corrosion cracking, especially in the severe corrosive environment of the refuse incinerator in Japan, It is an object of the present invention to provide a relatively inexpensive high corrosion-resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, which has further improved general corrosion resistance at high temperatures and resistance to intergranular corrosion.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies to achieve the above-mentioned object, and as a result, 1) in order to further improve general corrosion resistance especially at high temperature, an alloy having an austenite structure Then, Al addition is effective,
2) The knowledge obtained in the prior invention can be applied to alloys containing Al, and in order to reduce the stress corrosion cracking susceptibility of steel, the Mo content is set within the range of [5.8-Ni (%) / 10]% or less. It should be adjusted. 3) As described in the prior invention, when Mo is contained excessively, the alloy becomes more susceptible to stress corrosion cracking, and stress corrosion cracking is likely to occur. Since the intergranular corrosion is likely to occur if it is added, the present invention has been found out that it is necessary to contain at least 0.5% or more.

Here, the gist of the present invention is the following highly corrosion-resistant alloy steels for heat transfer tubes of waste incineration waste heat boilers.

% By weight, C ≦ 0.05%, Si ≦ 2%,
Mn ≦ 2.5%, 15% ≦ Cr ≦ 30%, 25% ≦ Ni ≦ 50%,
1% ≤ Al ≤ 5%, 0.5% ≤ Mo ≤ [5.8-Ni (%)
/ 10]%, the balance consisting of Fe and unavoidable impurities, high corrosion-resistant alloy steel for waste heat incinerator waste heat boiler heat transfer tubes.

[0009] In addition to the alloy components described above, a high corrosion resistance for waste heat incinerator waste heat boiler heat transfer tubes containing 0.1 to 3% by weight in total of one or more selected from Nb, Ti, Zr and V. Alloy steel.

In addition to the alloy components described above, a high corrosion resistant alloy for waste incineration waste heat boiler heat transfer tubes, which further contains 0.1 to 5% by weight in total of one or more selected from Cu, Co and W. steel.

In addition to the alloy components described above, a total of one or more selected from rare earth elements is added.
High corrosion resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers containing 01 to 0.1% by weight.

In addition to the alloy components described above, 0.1 to 3 wt% of one or more selected from Nb, Ti, Zr and V in total is selected from among Cu, Co and W. High corrosion resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers containing 0.1 to 5% by weight of one or more types in total.

In addition to the alloy components described above, a total of 0.1 to 3% by weight of one or more selected from Nb, Ti, Zr and V, and one or more selected from rare earth elements. High corrosion-resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers containing 0.01 to 0.1% by weight in total.

In addition to the alloy components described above, 0.1 to 5% by weight in total of one or more selected from Cu, Co and W, and one or more selected from rare earth elements in total High corrosion resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers containing 0.01 to 0.1% by weight.

In addition to the alloy components described above, a total of one or more selected from Nb, Ti, Zr and V is 0.1 to 3 wt%, and is selected from Cu, Co and W. 0.1-5% by weight in total for one or more, 0.01-0.1 in total for one or more selected from rare earth elements
High corrosion-resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers that contains wt%.

[0016]

The function of the alloy components of the steel of the present invention and the reasons for limiting their contents will be described below.

C: C is combined with Cr in the alloy and precipitates as lumpy Cr carbides at the grain boundaries to deteriorate high temperature strength, or the Cr carbides deposited at the grain boundaries adhere to the surface of the pipe. Since it may react with the molten salt compound to cause intergranular corrosion, its content is preferably as low as possible. If the content of C exceeds 0.05%, the above problem becomes remarkable, so the content was made 0.05% or less.

Si: Si is necessary as a deoxidizing agent, and is generally an effective element for enhancing the oxidation resistance.
However, in an alloy steel containing a considerable amount of Al such as the alloy steel of the present invention, if the Si content exceeds 2%, the crack susceptibility of the welded portion increases and the hot workability deteriorates. The content was 2% or less.

Mn: Mn is an austenite forming element and can also be used as a deoxidizing agent, so it is usually added to some extent. However, if the content exceeds 2.5%, the oxidation resistance and hot workability deteriorate, so the content was made 2.5% or less.

Cr: Cr has an excellent effect in improving high temperature strength and high temperature oxidation resistance, but if its content is less than 15%, sufficient improvement in high temperature oxidation resistance cannot be obtained. And On the other hand, in a corrosive environment where molten chloride adheres, such as in Japan's refuse incinerators, even if the amount added is increased, no significant effect is observed for improving corrosion resistance.
If more than 30% of Cr is contained, Cr oxide, which is originally protective, reacts with molten chloride to form volatile Cr ₂ O ₂ Cl _{2 and} deteriorates corrosion resistance. Therefore, the upper limit is 30%. did.

Ni: Ni is also an austenite forming element and is an important element for ensuring high temperature strength and suppressing general corrosion at high temperatures. However, in the alloy steel containing a considerable amount of Al such as the alloy steel of the present invention, the Ni content is 25
If less than 25%, the general corrosion resistance deteriorates rapidly, so 25%
Was set as the lower limit. On the other hand, Ni is an expensive alloying element, and the addition of more than necessary causes an increase in cost, so the upper limit was made 50% in consideration of cost and corrosion resistance.

Al: Al not only exhibits an excellent effect of improving high temperature oxidation resistance, but also in a corrosive environment of a refuse incinerator, that is, in a corrosive environment of a refuse incinerator where molten chloride adheres to the pipe surface. It is an important element for imparting excellent corrosion resistance. These effects of Al become remarkable when the content is 1% or more, but when the content exceeds 5%, precipitation of γ'-type Fe-Al intermetallic compound is remarkable when used for a long time at high temperature, and alloy steel Not only the toughness of the steel deteriorates, but also the hot workability deteriorates and cracks easily occur during forging. Therefore, the content is set to 1 to 5%.

Mo: Mo is an element that is usually added to improve the corrosion resistance in a wet corrosion environment, but if added in a large amount in the corrosive environment of a refuse incineration waste heat boiler in Japan, it becomes an element that enhances stress corrosion cracking susceptibility. .. In the steel containing Mo, the stress corrosion cracking susceptibility strongly depends on the Ni content in the alloy steel as described above, and the Mo content is [5.8-Ni
If it exceeds (%) / 10]%, stress corrosion cracking tends to occur. However, on the other hand, Mo has the effect of suppressing intergranular corrosion, and it is necessary to contain 0.5% or more in order to suppress intergranular corrosion at high temperatures. Therefore, if the Mo content is 0.5% or more, [5.8-
Ni (%) / 10]% or less.

Nb, Ti, Zr, V: Nb, Ti, Zr and V all easily form carbides, fix C in the alloy steel to suppress precipitation of Cr carbides, and suppress deterioration of high temperature strength. It is valid. Further, in the case of austenitic steel, Cr carbide precipitated at the grain boundaries reacts with corrosive molten chloride adhering to the surface of the pipe to cause intergranular corrosion. If added, the resistance to intergranular corrosion is further improved. If necessary, one or more of these elements can be added, but the total content of the one or more elements is 0.1%.
If it is less than the above, the effect of addition does not appear, and if the total content of one or more exceeds 3%, the effect of addition is saturated and only the cost is increased. Therefore, when adding these elements, the content is preferably 0.1 to 3% in total of one or more kinds.

Cu, Co, W: Each of Cu, Co and W has the effect of improving the high temperature strength of the alloy steel through solid solution strengthening. Therefore, in order to further increase the high temperature strength, one or more kinds are added. May be. However, if the total content of one or more of these elements is less than 0.1%, the effect of addition is not remarkable, and the effect of addition is saturated even if the total content of one or more elements exceeds 5%. However, since only the cost will increase, when it is added, the content is preferably 0.1 to 5% in total of one or more kinds.

Rare Earth Elements: Rare earth elements such as Y, La and Ce improve the adhesion of (Cr, Al) ₂ O ₃ of the protective oxide film formed on the alloy surface,
Since it has the function of improving the high temperature oxidation resistance, one or more kinds may be added to enhance the high temperature oxidation resistance. The above effects are more remarkable than 0.01% in total of one or more, but 0.1
If it exceeds%, the hot workability of the material will deteriorate.
Therefore, when one or more rare earth elements are added, the content of one or more rare earth elements is preferably 0.01 to 0.1% in total.

[0027]

[Examples] Alloy steels having the chemical compositions shown in Table 1 (1) to (4) were melted in a vacuum melting furnace at a rate of 17 kg, forged into ingots, then heated to a temperature of 1200 ° C, hot forged and heat-treated. It was rolled into a billet with a thickness of 15 mm. Next, these billets were soft-annealed at 1100 ° C. and then cold-rolled into a steel plate having a thickness of 10.5 mm. After that, solution heat treatment was performed by heating to 1200 ° C. and then water cooling. In the hot forging, steels 67 and 69 were markedly cracked on the end face of the billet during hot forging, but other steels could be forged without any problems.

After the solution heat treatment, a corrosion test piece having a thickness of 2 mm, a width of 10 mm and a length of 10 mm and a stress cracking test piece having the dimensions and shapes shown in FIG.
A corrosion test simulating a refuse incinerator atmosphere was conducted.

The high temperature corrosion test is carried out in terms of mol% on both sides of the test piece.
10% NaCl-10% KCl -15 % FeCl 2 -15% PbCl 2 - 18.75
% Na ₂ SO ₄ − 18.75% K ₂ SO ₄ −12.5% Fe ₂ O ₃ synthetic ash 30
Apply at a ratio of mg / cm ² and apply 0.15% HCl-300ppmSO
₂ - 7.5% O ₂ - with heating in _{7.5% CO 2 -20% H 2} O -bal.N 2 of the temperature of the gas stream 550 ° C., a test of heating 20 hours. The corrosion weight loss was determined by descaling the corrosion test piece after the high temperature corrosion test and measuring the weight, and determining the weight change before and after the test. The presence or absence of intergranular corrosion was examined by microscopically observing the surface of the above-described descaled corrosion test piece with an optical microscope at a magnification of 100 times and, if necessary, 500 times.

In the stress corrosion cracking test, as shown in FIG. 2, a stress of about 0.2% of alloy steel is applied to the stress cracking test piece 2 with the jig 1, and in this state, the surface of the test piece 2 is subjected to the above high temperature. After applying the same synthetic ash used in the corrosion test, while heating to a temperature of 400 ° C in a gas stream of the same gas composition for 20 h
It is a test to hold. The test temperature was 400 ° C. because of the finding that the stress corrosion cracking susceptibility of the alloy steel having an austenitic structure is most remarkable at 400 ° C. The presence or absence of stress corrosion cracking was examined by microscopically observing the cross section of the semicircular notch. These results are summarized in Table 2 (1).
~ (4).

[0031]

[Table 1 (1)]

[0032]

[Table 1 (2)]

[0033]

[Table 1 (3)]

[0034]

[Table 1 (4)]

[0035]

[Table 2 (1)]

[0036]

[Table 2 (2)]

[0037]

[Table 2 (3)]

[0038]

[Table 2 (4)]

In Table 1, reference numerals 1 to 62 are steels of the present invention, reference numerals 63 to 71 are comparative steels, and reference numerals 73 to 78 are existing steels. Among the existing steels, the steel with the code 72 is NO8825 described in ASTM B163.
Alloy, code 73 is SUS304, code 74 is SUS316L, code 75 is SU
S310S and reference numeral 76 respectively correspond to NO8320 steel described in ASTM B622, and reference numerals 77 to 78 are prior invention steels.

As shown in Table 2, the steel of the present invention is excellent in general corrosion resistance, intergranular corrosion resistance and stress corrosion cracking resistance.

The steels of the prior inventions of reference numerals 77 to 78 have a smaller corrosion weight loss than the existing steels of the reference numerals 72 to 76, but the steels of the present invention of reference numerals 1 to 62 have a smaller corrosion weight loss than the steels of the prior application and have a corrosion resistance. Is much better. Further, the steel of the present invention has a grain boundary corrosion depth of 20 μm or less, and no stress corrosion cracking is observed. This is because the stress corrosion cracking susceptibility was sufficiently lowered as a result of adjusting the Mo content.

The steels with the reference numerals 64, 65 and 66 in which the Mo content is higher than the range specified in the present invention all have stress corrosion cracking, and the steels with the reference numerals 63, 73, 75 and 78 which do not contain Mo. The grain boundaries of all steels are eroded by corrosion,
It can be seen that is required to be contained within an appropriate range.

In the steel having a large C content, code 68, intergranular corrosion is observed even though it contains a predetermined amount of Mo. This is because Cr carbide precipitated at the grain boundaries and reacted with molten chloride. No remarkable intergranular corrosion is recognized in the steel of code 2 containing 0.04% of C, so that it is necessary to set C to 0.05% or less.

The steel with a large Cr content of 70, which has no stress corrosion cracking, has a large corrosion weight loss because it is a high Cr steel. In addition, this steel also exhibits remarkable intergranular corrosion and is poor in corrosion resistance despite containing Al.

Since existing steel SUS304 (reference numeral 73) has a low Ni content, stress corrosion cracking is recognized and the amount of general corrosion is large. In addition, existing steels such as SUS316L (reference numeral 74) and SUS310S (reference numeral 7)
In 5), although the sensitivity to stress corrosion cracking is low, the amount of general corrosion is extremely large and the corrosion resistance in the corrosive environment of the refuse incinerator is not good. On the other hand, the steel of No. 72 (NO8825 alloy) has a small amount of general corrosion, but stress corrosion cracking occurs because it contains Mo of about 2.5%.

It can be seen that the steel of reference numeral 71 containing only 0.4% of Al has a slightly larger corrosion weight loss than the steel of the present invention, and it is necessary to contain Al of 1% or more. However, 5%
The alloy with the code 67 containing Al in excess of 60 ° C markedly deteriorates the toughness at 0 ° C after aging treatment at 550 ° C for 300 hours (the metal using the No. 4 test piece specified in JIS Z 2202). Charpy impact value by material impact test method is 3kgfm / c at 0 ℃
m was found to be unsuitable as a material for less than ²⁾ heat transfer boiler heat pipe. It was also found that this steel also has a problem in hot workability. All of the steels of the present invention showed a Charpy impact value of ³ kgfm / cm ² or more even after such aging treatment, and no remarkable decrease in toughness was observed.

From the results of the examples, the steel of the present invention exhibits good corrosion resistance performance even in a high temperature part which is a very severe corrosive environment among the refuse incinerators, and since it has sufficient high temperature strength, it is sufficiently applied to such a part. It was confirmed that it was possible.

[0048]

The present invention is to provide a heat transfer tube material for a boiler having an austenite structure, which has excellent corrosion resistance in a special and extremely severe corrosive environment such as a refuse incinerator atmosphere.

Since this steel has an austenitic structure, it has high temperature strength, workability and weldability.
Moreover, since the Ni content is 50% or less, it can be said to be a relatively inexpensive material. By using the pipe made of the steel of the present invention at a high temperature part, for example, a superheater pipe, the waste heat of waste incineration can be fully utilized, and energy can be efficiently extracted as electric power from municipal waste, which is a valuable resource. Is possible.

[Brief description of drawings]

FIG. 1 is a plan view and a side view showing a shape of a corrosion test piece used for measuring a corrosion weight loss in a high temperature corrosion test.

FIG. 2 is a schematic view showing a jig used in a stress corrosion test.

[Explanation of symbols]

 Reference numeral 1 is a jig, and 2 is a stress cracking test piece.

Claims (8)

[Claims] 1. By weight%, C ≦ 0.05%, Si ≦ 2%, Mn
≤ 2.5%, 15% ≤ Cr ≤ 30%, 25% ≤ Ni ≤ 50%, 1%
≤ Al ≤ 5%, 0.5% ≤ Mo ≤ [5.8-Ni (%) / 1
0]%, and the balance being Fe and unavoidable impurities, a highly corrosion-resistant alloy steel for waste heat incinerator waste heat boiler heat transfer tubes. 2. Garbage containing 0.1 to 3% by weight in total of one or more selected from Nb, Ti, Zr and V in addition to the alloy components described in claim 1. High corrosion-resistant alloy steel for incinerator waste heat boiler heat transfer tubes. 3. Waste incineration waste, characterized in that, in addition to the alloy components described in claim 1, it further contains one or more selected from Cu, Co and W in a total amount of 0.1 to 5% by weight. High corrosion resistant alloy steel for heat boiler heat transfer tubes. 4. In addition to the alloy components described in claim 1, at least one selected from rare earth elements is added in a total amount of 0.1.
High corrosion-resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, characterized by containing 01 to 0.1% by weight. 5. In addition to the alloy components described in claim 1, 0.1 to 3% by weight in total of one or more selected from Nb, Ti, Zr and V among Cu, Co and W. A high corrosion-resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, which contains 0.1 to 5% by weight in total of one or more selected from the above. 6. In addition to the alloy components described in claim 1, one or more selected from Nb, Ti, Zr and V in total of 0.1 to 3% by weight, selected from rare earth elements. A high corrosion-resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, which contains 0.01 to 0.1% by weight of one or more kinds in total. 7. In addition to the alloy components described in claim 1, 0.1 to 5% by weight in total of one or more selected from Cu, Co and W, and 1 selected from rare earth elements. High corrosion-resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, characterized in that it contains 0.01 to 0.1% by weight in total of more than one kind. 8. In addition to the alloy components described in claim 1, 0.1 to 3% by weight in total of one or more selected from Nb, Ti, Zr and V among Cu, Co and W. 0.1 to 5% by weight in total for one or more selected from 0.01 to 0.1 in total for one or more selected from rare earth elements
High corrosion resistant alloy steel for heat transfer tubes of waste incineration waste heat boilers, characterized by containing wt%.