JPH09279293A - Steel excellent in exhaust gas corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a steel having good corrosion resistance to the dew point corrosion by an exhaust gas used for the flue or chimney of a combustion exhaust gas in a thermal power plant or the like using LNG as the main fuel. SOLUTION: This steel has a compsn. contg., by weight, <=0.15% C, <=1.0% Si, <=2.0% Mn, 0.5 to 6% Cr, 0.1 to 1.0% Cu, 0.05 to 1.0% Ni, 0.02 to 1.0% Mo, <=0.1% Al and 0.001 to 0.05% N, furthermore contg., at need, one or >=two kinds among <=0.2% Sb, <=0.1% Sn and <=0.1% Pb, and the balance Fe with inevitable impurities. Furthermore, the steel satisfies the inequality I, DW =0.7Cr+Cu+2Ni+Mo+5Sb+6Sn+5Pb>1.0 and the inequality II, PC=C+Si/30+Mn/20+Cr/ 20+Cu/20+Ni/60+Mo/15+Sb/40+Sn/40+Pb/60<0.4 (where, each elemental symbol denotes the weight% of the element, and the elements not to be added are treated as zero).

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a thermal power plant using LNG as a main fuel (hereinafter abbreviated as thermal power plant).
The present invention relates to a steel used for a flue of a combustion exhaust gas (hereinafter, abbreviated as exhaust gas) or a chimney and having good corrosion resistance against dew point corrosion of the exhaust gas.

[0002]

2. Description of the Related Art CO _{2 in} exhaust gas from a thermal power plant,
NO _x , SO _x, etc. are contained, and in most cases, when they become an aqueous solution, they become strongly corrosive acids. Therefore, in the constituent materials of the chimney and the flue through which the exhaust gas flows, dew point corrosion due to the condensation of water in the exhaust gas has become a problem.

In these exhaust systems, that is, in the inner tube materials that come into contact with exhaust gas such as flues, chimneys, and smoke ducts, particularly high reliability is required in a portion requiring general corrosion resistance or crevice corrosion resistance. Therefore, austenitic stainless steel such as SUS304 has been used.

Further, in a heavy oil fired boiler or a coal-fired boiler in which an exhaust gas having a higher corrosiveness flows, the SU
Higher Cr content than S304 steel, eg 25Cr
Austenitic stainless steel may also be used.

On the other hand, in recent thermal power plants, L
There is a tendency to promote the use of fuel that has abundant resources such as NG and has a clean exhaust gas. In addition, because it is necessary to take environmental measures in plants using conventional fuel,
Denitrification, providing the facilities of the desulfurization etc. are also generalized, the amount of harmful components such as results in NO _X, SO _X in many plants has dropped to 1000ppm or less.

In plants where these exhaust gases are relatively clean, the high corrosion resistance of austenitic stainless steel such as SUS304 is not necessarily required, and 17Cr ferritic stainless steel such as SUS430 is also used. .

As a material which can be used in such a mild corrosive environment, for example, Japanese Patent Laid-Open Nos. 6-192788 and 6-192789 disclose low C-
Low Mn-P-Cu-Ni steel, and low C-low Mn-Cu
-Ni-Mo steel is disclosed.

[0008]

However, of the techniques presented above, stainless steel is expensive. Further, austenitic stainless steel generally has a problem of stress corrosion cracking, and considerable cost and labor are required to avoid it.

Ferritic stainless steel is also SUS30
It is not necessarily cheaper than austenitic stainless steels such as No. 4 and the like, and this type of stainless steel also has drawbacks such as low toughness of the welded portion and is not an effective solution.

On the other hand, the low C-low Mn-P-Cu-Ni steel and the low C-low Mn-Cu-Ni-Mo steel are not yet sufficient in alloy amount for ensuring corrosion resistance. Due to the above reasons, thermal power plants that use LNG as the main fuel,
Although the use of other fuels, NO _X in the exhaust gas, SO _X
In a thermal power plant with a low concentration of, the development of a steel that has an appropriate balance of corrosion resistance against exhaust gas and economic efficiency and is easy to use has been awaited.

[0011] The biggest problem actualized in steel used in the exhaust gas environment of such thermal power plants, the dew point corrosion of exhaust gas containing CO ₂ (hereinafter referred to as CO ₂ corrosion.) A material damage. The CO ₂ concentration in the exhaust gas varies depending on the fuel, but is usually about 10%. The same applies to LNG-only burning plants, and steel that can withstand this is required.

By the way, the chromium oxide layer (passive film) formed on the surface of the steel is chemically stable and effective in corrosion resistance, and improves the corrosion resistance in a CO ₂ corrosive environment. It is well known that a certain amount or more of Cr must be added in order to form this chromium oxide layer on the surface of steel. However, in the case of low alloy steel, there are very few examples of investigating the relationship between the added amount of Cr and the corrosion resistance in the exhaust gas environment, unlike the case of stainless steel.

The optimization of the amount of Cr added to obtain corrosion resistance in an exhaust gas environment, the effect of other elements that complement or substitute the effect of Cr, etc. have heretofore been treated as unspecified.

On the other hand, an increase in the Cr content causes a problem that the weldability is deteriorated. Further, the influence of other elements that complement or substitute the effect of Cr on the weldability from the viewpoint of corrosion resistance is not clear.

The present invention has been made in view of the above circumstances, and an object thereof is to dew-point corrode exhaust gas used for a flue or a stack of combustion exhaust gas of a thermal power plant using LNG as a main fuel. To provide a steel having good corrosion resistance to.

[0016]

In order to achieve this object, the inventors of the present invention have proposed that if the environmental conditions such as CO ₂ , NO _x and SO _x concentrations change in a wide range and are not specified, these The anticorrosion effect of Cr and added elements in the environment changes variously, and its effectiveness is not clear, but when the environmental conditions are specified within a certain range, the effective range of the effective added element and its addition amount should be set to the appropriate range. I proceeded with the research from the viewpoint that it could be specified.

Considering that the increase of Cr content deteriorates the weldability and the effect of the elements effective for corrosion resistance on the weldability, the proper content of Cr and other elements is also considered from the viewpoint of weldability. I proceeded with the research from the viewpoint that it should exist.

First, it is assumed that the exhaust gas when LNG is used as the main fuel and the exhaust gas that uses other fuels complete with facilities for denitration, desulfurization, etc. are relatively clean.
CO ₂ is less than 20%, NO _X is less than 50 ppm, SO _X
It was found that the effect of the additional element is clear under the environmental condition of less than 0.1%.

Therefore, in order to clarify the effective elements including the combination of the elements in the above environment and the appropriate addition range thereof, the inventors of the present invention melt-produced steels having various compositions, After rolling, a general corrosion resistance and a local corrosion resistance in a CO ₂ corrosive environment were investigated.

As a result, the additive elements effective for the general corrosion of steel in a CO ₂ corrosive environment such as exhaust gas using LNG as the main fuel are Cr, Cu, Ni, Mo, Sb, Sn and P.
It has been found that Mo and N are effective against local corrosion.

Further, the influence of these elements and the elements generally contained in steel on the weldability, especially on cold cracking was clarified, and the present invention was completed. That is, the present invention is, by weight%, C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 0.5 to 6%, Cu:
0.1-1.0%, Ni: 0.05-1.0%, Mo:
0.02-1.0%, Al: 0.1% or less, N: 0.0
01-0.05%, Sb: 0.2% or less (including 0%), Sn: 0.1% or less (including 0%), Pb: 0.
Steel containing 1% or less (including 0%), the balance being Fe and unavoidable impurities, and further satisfying the formulas (1) and (2), which is excellent in exhaust gas corrosion resistance. . DW = 0.7Cr + Cu + 2Ni + Mo + 5Sb + 6Sn + 5Pb> 1.0 ... (1) PC = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + Sb / 40 + Sn / 40 + Pb / 60 <0.4 ... (2) However, (1) Formula and Formula. In the formula (2), each element symbol represents the weight% of the element, and the element not added is treated as 0.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the additive elements in the present invention will be described below. C is an element that gives strength to the steel, but if added in excess of 0.15%, the weldability and general corrosion resistance and local corrosion resistance in a CO ₂ corrosive environment are deteriorated, so the addition amount is 0. 15% or less.

Si is an important element for deoxidizing steel,
If added in excess of 1.0%, the weldability deteriorates, so the upper limit of addition is set to 1.0%. Mn contributes to the strength of the steel, but if added over 2.0%, the weldability and CO ₂
In order to deteriorate the general corrosion resistance and the local corrosion resistance in a corrosive environment, the addition amount thereof is set to 20% or less.

Cr is a very effective additive element from the viewpoint of ensuring general corrosion resistance in a CO ₂ corrosive environment. The effect becomes remarkable at 0.5% or more. On the other hand, if it exceeds 6%, the weldability is significantly deteriorated, so the range of addition is 0.5 to
6%.

Cu is an element that contributes to the improvement of general corrosion resistance in a CO ₂ corrosive environment. If the effect is less than 0.1%, it is not sufficient. On the other hand, if added in excess of 1.0%, the weldability deteriorates, so the addition amount is 0.1-1.0%.
And

Ni is an element effective for improving general corrosion resistance in a CO ₂ corrosive environment. It is also an important additional element for preventing Cu cracking. It is a relatively expensive element and it is desirable to add a small amount from the economical viewpoint, but if it is less than 0.05%, its effect is not sufficient. On the other hand, even if added over 1.0%, the effect is saturated, so the range of addition is set to 0.05 to 1.0%.

Mo is an effective additive element for general corrosion resistance and local corrosion resistance in a CO ₂ corrosive environment, but the effect is not clear at less than 0.02%. On the other hand, if added over 1.0%, a large amount of carbide is generated,
Since the ductility and weldability will be problematic, the addition amount is made 0.02 to 1.0%.

Al, like Si, is an important element for deoxidizing steel, but if added in excess of 0.1%, the ductility and weldability will deteriorate, so the addition amount should be 0.1% or less. .
N is contained in steel in a small amount to improve strength characteristics, and CO ₂
It is an additive element effective for local corrosion resistance in a corrosive environment, but if it is less than 0.001%, it has no effect. Also,
If the content exceeds 0.05%, the weldability and hot workability deteriorate, so the content is 0.001-0.0
5%.

Sb is an element effective for improving the general corrosion resistance in a CO ₂ corrosive environment, but if it is added in an amount exceeding 0.2%, ductility and weldability deteriorate, so the upper limit of the addition amount is 0. 0.2%. Sn is also an element effective in improving general corrosion resistance in a CO ₂ corrosive environment, but if added in excess of 0.1%, ductility and weldability deteriorate, so the upper limit of addition is 0.1%. And Pb, like Sb, has CO ₂
It is an element effective for improving the general corrosion resistance in a corrosive environment, but if added in excess of 0.1%, ductility and weldability deteriorate, so its addition amount is set to 0.1% or less.

In addition to the following restrictions, DW = 0.7Cr + Cu + 2Ni + Mo +
It is assumed that 5Sb + 6Sn + 5Pb> 1.0 (1) is satisfied. (However, elements that are not added are treated as zero.) If the above formula is not satisfied, the amount of effective elements will be insufficient, and thus the general corrosion resistance in a CO ₂ corrosive environment will not be sufficient.

Further, it is assumed that PC = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + Sb / 40 + Pb / 60 <0.4 (2). (However, elements that are not added are treated as zero.) If the above formula is not satisfied, cold crack resistance in welding is significantly deteriorated, and high-temperature prognosis is required during welding work, which significantly impairs weldability.

The present invention allows unavoidable impurities to the extent that they are usually contained in steel. For example, P and S are impurities that deteriorate the hot workability of steel, and the smaller the amount, the better, but both elements may be contained in an amount of about 0.03.

[0033]

Next, an embodiment of the present invention will be described. Table 1
50 kg of the present invention steel having the composition shown in Table 1 and comparative steel
After being melted and cast in an induction heating melting furnace, it was soaked at 1150 ° C. and rolled to a thickness of 15 mm by a hot rolling mill. The rolling finishing temperature is set at 850 ° C., and water cooling was performed immediately after rolling to cut out test pieces.

The corrosion test was carried out by immersing in pure water at 80 ° C. in a saturated solution having gas bubbling. Gas composition is 1
It is 0% CO ₂ balance N ₂ gas, and simulates the exhaust gas of LNG exclusive combustion.

In the welding test, an oblique Y-shaped weld cracking test was carried out, and the low temperature cracking susceptibility was evaluated by the preheating temperature at which cracking did not occur in the heat affected zone of the base material. The welding material is a low hydrogen type coating material (D4316) defined in JIS Z 3211, and the core wire is JIS G 3
One corresponding to one of 523 (SWY11) was used.
The heat input was 17 kJ / cm.

In the table, 1 to 17 are the steels of the present invention, and 18 to
26 is a comparative steel. FIG. 1 shows the effects of these elements on the corrosion resistance arranged by the DW values shown above. In the figure, the horizontal axis represents the DW value and the vertical axis represents the corrosion weight loss. (In the figure, the results of steels not shown in Table 1 are also shown, and they do not correspond exactly to the steels of Table 1.) The corrosion weight loss shows a decreasing tendency as the DW value increases, DW
It can be seen that when the value is 1.0 or less, the corrosion weight loss is relatively large.

Next, FIG. 2 shows the effects of these elements on the weldability arranged by the PC values shown above. In the figure, the horizontal axis represents the PC value, and the vertical axis represents the preheating temperature at which cracking does not occur in the welding test. (Note that
In the figure, the results of the steels not shown in Table 1 are also shown, and the steels in Table 1 do not correspond exactly. ) It can be seen that the preheating temperature at which cracking does not occur tends to rise as the PC value increases, and rises sharply when the PC value exceeds 0.4.

[0038]

[Table 1]

[0039]

INDUSTRIAL APPLICABILITY The steel of the present invention exhibits good corrosion resistance as a flue and a chimney of a thermal power plant that uses a relatively clean fuel such as one containing LNG as a main component. Further, when components other than LNG, that is, heavy oil, naphtha, coal, etc. are co-firing, as long as they are limited within a certain range, the same is true, and also in the case of a thermal power plant that uses a fuel other than LNG, When the exhaust gas is relatively clean, it is expected to show excellent corrosion resistance as well.

Further, by appropriately controlling the amount of alloy, the cold cracking susceptibility during welding can be suppressed to a low level,
It has excellent weldability because it does not require prognostic heat at high temperatures.

In terms of practicality, the steel of the present invention is economical because it contains less alloying elements than stainless steel, stainless clad steel, etc., which have many restrictions on use and are expensive. It is also an extremely useful steel from the viewpoint of resource saving.

The steel of the present invention includes steel plates and strips obtained by plate rolling, hot strip mill rolling, etc., seamless steel pipes such as the Mannesmann method, the Eugene Sejournet method, electric resistance welded pipes, spiral steel pipes, UOE steel pipes, etc. All of the hot rolled steel products such as various welded steel pipes, bar steels, and bar steels, as well as cold, warm rolled and processed products thereof, exhibit the above-mentioned characteristics, regardless of the manufacturing method and product shape. Absent.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a corrosion weight loss and a DW value in a corrosion test simulating the dew point corrosion of an LNG-exhausted exhaust gas.

FIG. 2 is a diagram showing a relationship between a PC value and a preheating temperature at which cracking does not occur in a diagonal Y-type weld crack test.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] September 12, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction contents]

Further, in a heavy oil fired boiler or a coal-fired boiler in which an exhaust gas having a higher corrosiveness flows, the SU
Higher Cr content than S304 steel, eg 25Cr
Austenitic stainless steel may also be used.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0030

[Correction method] Change

[Correction contents]

In addition to the above-mentioned limitations, DW = 0.7Cr + Cu + 2Ni + Mo +
It is assumed that 5Sb + 6Sn + 5Pb> 1.0 (1) is satisfied. (However, elements that are not added are treated as zero.) If the above formula is not satisfied, the amount of effective elements is insufficient, so that general corrosion resistance in a CO ₂ corrosive environment is insufficient.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0031

[Correction method] Change

[Correction contents]

Further, it is assumed that PC = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + Sb / 40 + Sn / 40 + Pb / 60 <0.4 (2). (However, elements that are not added are treated as zero.) If the above formula is not satisfied, cold crack resistance in welding is significantly deteriorated, and high-temperature prognosis is required during welding work, which significantly impairs weldability.

Claims (1)

[Claims] C. 0.15% or less by weight, Si:
1.0% or less, Mn: 2.0% or less, Cr: 0.5 to 6
%, Cu: 0.1 to 1.0%, Ni: 0.05 to 1.0
%, Mo: 0.02-1.0%, Al: 0.1% or less,
N: 0.001 to 0.05%, Sb: 0.2% or less (0
%), Sn: 0.1% or less (including 0%), P
b: 0.1% or less (including 0%), the balance consisting of Fe and unavoidable impurities, and further excellent in exhaust gas corrosion resistance characterized by satisfying formulas (1) and (2) Steel. DW = 0.7Cr + Cu + 2Ni + Mo + 5Sb + 6Sn + 5Pb> 1.0 (1) PC = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + Sb / 40 + Sn / 40 + Pb / 60 <0.4 (2) However, the formula (1) and In the formula (2), each element symbol represents the weight% of the element, and the element not added is treated as 0.