JPS6160849A - High strength alloy having high corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a forgeable alloy steel having superior strength at high temp. and superior corrosion resistance and suitable for use as a material for the boiler tube of a boiler for thermal-power generation using coal as fuel by adding specified amounts of Al and Ti to an Ni-Cr high-alloy steel having an austenite structure. CONSTITUTION:In a steam boiler for thermal-power generation using coal as fuel, the temp. of combustion gas is 600-800 deg.C and the temp. of steam is 600-700 deg.C. A high-alloy steel contg. 0.03-0.15% C, 55-65% Ni, 28-32% Cr, 0.7-1.2% Mn, 0.7-1.2% Si, 0.8-1.6% Mo, 0.6-1.5% Nb, 0.3-0.7% Al and 0.6-2.0% Ti (Al/Ti=0.2-0.5 by weight) and having an austenite structure is used as a material for the boiler tube of the steam boiler. The alloy steel has especially superior strength at high temp. and superior corrosion resistance because the steel contains Al and Ti.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a heat-resistant alloy with improved high-temperature creep rupture strength by improving the highly corrosion-resistant alloy Inconel 690. A forgeable, high-strength, forgeable material for boiler tubes that is suitable for use under high temperature and high pressure conditions, where the outside combustion gas temperature is approximately 600 to 800C and the inside steam temperature is approximately 600 to 700C.
Concerning highly corrosion resistant alloys. The present invention also relates to an alloy applicable to structural materials for molten carbonate fuel cells and structural materials for chemical plants.

[Background of the invention]

Conventional coal-fired boiler tubes, especially superheater tubes, are made of materials that are used in a severely corrosive environment due to combustion gases and attached ash (mainly alkali salts). The action often leads to destruction. The main causes of this corrosion thinning are the Sow contained in the combustion gas 3 and the low melting point NazSO4 contained in the combustion ash.
, KxSO4 attacks the outer wall of the tube. On the other hand, recent thermal power plants
In order to conserve resources, save energy, and improve transmission efficiency, the shift from conventional heavy oil combustion to coal combustion and higher steam conditions at higher temperatures and pressures are being strongly promoted, making the corrosive environment described above even more severe. There is a demand for materials that not only have high temperature strength, but also have high temperature strength υ higher than conventional ones. In conventional coal-fired boilers, the maximum outer wall temperature is around 600C,
Since the steam pressure was around 246KII/-, 5US
300 series stainless steel, especially 8U8316, 5U832
1°5 US347 etc. were used. In these plants, destruction occurred mainly due to thinning due to corrosion and steam oxidation during long-term use, and damage due to creep and fatigue added to this. Conventional countermeasures against corrosion caused by combustion gas and attached ash include applying chromizing treatment to the outer wall of the tube, forming a dense and stable film with good adhesion by adding rare earth elements, and using alkali salts with a low melting point. We are dealing with this by increasing the Further, as a countermeasure against steam oxidation of the inner wall of the tube, shot peening or the like is performed to make the crystal grains on the steam contact surface finer. However, when looking at chromizing treatment and shot peening, for example, there is a problem of reduced effectiveness due to construction techniques and welding when applied to actual plants. Furthermore, as the usage environment has become harsher due to higher temperature and higher pressure steam conditions, the material itself needs to have excellent high temperature gas corrosion resistance and steam oxidation resistance. Furthermore, as the temperature and pressure increase, the design strength relative to the tube design dimensions also increases, so high-temperature strength becomes an important factor as well as the above-mentioned corrosion resistance.

Therefore, Ni has both high temperature strength and corrosion resistance.

Alloys with high Cr content are required. As such an alloy, 3uFertherm (27C) is an iron-based alloy.
r-3'6Ni-15Co), Incoloy 800 (2
1Cr-32Ni-0, 4AA-0, 4Ti), etc., but the former has problems in processing and welding into tubes9, and the latter is slightly lacking in strength. Also,
Many of the Ni-based alloys and CO-based alloys used in gas turbine materials are cast steel, which not only makes it difficult to manufacture seamless pipes by drawing, but also makes it difficult to produce seamless pipes by drawing.
'Welding is not possible in precipitation-strengthened alloys.

However, as a high-strength, high-corrosion-resistant alloy, it has a high Cr content, has an austenitic structure that is stable even after long-term heating at high temperatures, and can be used at sufficiently high temperatures for drawing, bending, and solution treatment in consideration of weldability. Materials that provide strength are needed. High C
As an example of an r-i'J i alloy having strength and corrosion resistance, as shown in Japanese Patent Application Laid-Open No. 52-92818, the weight ratio of Cr is 10 to 40%, and Al is 0.5 to 5%.
%, Ni3 (A
In other words, there are alloys that gain strength by aging the r' phase, but these types of r' precipitation-strengthened alloys are unsuitable for boiler tubes because of the problems mentioned above in the construction process. It is. Also, JP-A-50-12
4822 "Ni- with excellent heat resistance and corrosion resistance"
Cr alloy is a Ni-based alloy that has a high Ti and Al content of 1.8 to λ8%, and also contains 16 to 24% CO, and like the former, it is an age-precipitation-strengthened alloy, so it is not suitable for use as a crimp tube. Appropriate. The "chromium-nickel alloy" disclosed in JP-A No. 47-22823 contains 40 to 55% Cr, Nb, and Ta.

This alloy is a nine-Ni based alloy with the addition of Ti.This alloy has nitride-forming elements added to suppress the decrease in ductility at room temperature after long-term heating at high temperatures.1. As such, it is difficult to apply. Furthermore, the "heat-resistant alloy" shown in JP-A-52-68021K is Ni
Contains 24-53% of Cr, 20-44% of Cr, C and S
It is a high centrifugally cast tube that is used for pipes used in petrochemical factories, and it is difficult to apply it to boiler tubes. As mentioned above, in terms of strengthening factors, high Cr-4Ji alloys are mainly centrifugal cast alloys with high Cat or γ' precipitation strengthened alloys with high Al and Ti additions. It is difficult to use as a material for boiler tubes due to problems such as compatibility and welding. Therefore, Inconel 690 is currently most suitable as a high-Or boiler tube material, but it has the problem of low high-temperature strength, particularly low creep rupture strength.

[Purpose of the invention]

The purpose of the present invention is to improve creep rupture strength and corrosion resistance,
The object of the present invention is to provide a forgeable high-strength, high-corrosion-resistant alloy suitable for boiler tubes for thermal power generation using coal as fuel.

[Summary of the invention]

In the present invention, C: 0.03 to 0.15% by weight.

Ni: 55-65%, Cr: 28-32% Mrl:
0.7-1.2%, s'i: 0.7-1.2%, M
o: 0.8-1.6%, Nb: 0.6-1.5%, Al
: 0.3-0.7%, 'pi: 0.6-10%
and the remainder F6, the (A A / T r ) weight ratio is 0.2 to 0.5, and it has an entirely austenitic structure. It is a high-strength, high-corrosion-resistant alloy that can be forged. In addition, other inventions are ht in the above specified invention.
/Tr: limited to 0.2゛~0.5, Z
'1) (0.0 endogeneity of at least one type of f, 'l'a)
It contains 3 to 0.5%. High strength according to the present invention
The reason for limiting each component element for the highly corrosion resistant alloy will be explained.

C is an austenite-forming element, and is effective in maintaining high-temperature strength by being solid-solved in the matrix. Also, M
C, Nb, Ti, etc., form carbides and precipitate within the grains, improving high-temperature strength. However, if the C content is less than 0.03%, this effect is slightly compromised and σ phase is generated, resulting in embrittlement. , the creep rupture strength at high temperatures for long periods of time is also significantly reduced.

Moreover, when it exceeds 0.15%, carbides precipitate at grain boundaries, aggregate and coarsen, resulting in a decrease in high-temperature strength and poor workability.

Impairs weldability. Furthermore, Cr, which is effective in improving corrosion resistance, is
Since ruCs is fixed as a deaf carbide, the amount of solid solution Cr decreases, and corrosion preferentially progresses from the Cr-deficient phase near the grain boundaries.

Based on the above, the C content was set to 0.03 to 0.15%. Especially KO is preferably 05 to 0.12%.

Ni coexists with Cr to improve heavy workability, stabilizes the austenite structure, and maintains high-temperature strength by forming a solid solution in the matrix. K to obtain a complete austenite structure is Ni
According to the 7-factor diagram calculated from the relational expression between equivalent weight and 1t of Cr, about 28% or more is required. The more Ni is contained, the better the corrosion resistance is, so if the total amount of Cr and Ni is about 9
It has been adjusted to be 0%.

In addition, in the p e -Cr -pJ i system, when the amount of Cr increases, a σ phase is generated in the range of 600 to 5 ooc, elongation,
Squeezing and creep strength is reduced. Even in a two-phase alloy of austenite and ferrite (γ+α) or complete austenite (γ), a σ phase is generated. The amount of Cr is 3
When the amount of Ni is kept constant at 0%, the generation of the σ phase tends to be suppressed as the amount of Ni increases.

When the Cr content is kept constant at 30% and the Ni content is varied from 20 to 70%, the larger the Ni content, the greater the effect of improving corrosion resistance, but from the viewpoint of suppressing the formation of the σ phase, about 60% is preferable.

Therefore, if the Ni content is less than 55%, the effect of suppressing the formation of the σ phase is small, and in relation to the Cr content, the Ni content is less than 55%.
~65%. In particular, 58 to 62% is preferable.

Furthermore, the present invention is characterized in that by adding ht and 'ri in combination, N is (A4
T i ) The purpose is to precipitate an intermetallic compound, that is, the γ' phase, thereby improving high temperature strength, particularly creep rupture strength. Adults in the γ' phase are affected by the amount of Al and 'ri added.

Cr is an element that is preferentially oxidized to form a protective film with good adhesion on the surface and is effective in significantly improving oxidation resistance.

In order to obtain sufficient oxidation resistance, at least 28% or more is required, and from the viewpoint of suppressing the formation of the σ phase, it is not preferable to add more than 32%.

11Ji amount is constant at 40% and Cr amount is 20 to 50%.
From the results of investigating the relationship between the Cr content and oxidation resistance when the content is varied within the range of 28% or more, the Cr content must be 28% or more in order to improve the oxidation resistance. However, a Cr content exceeding 32% is undesirable because the formation of the η phase is promoted.

Si has a deoxidizing effect and is a necessary element as a deoxidizing agent, as well as an element that improves corrosion resistance. However, if too much is added, it will promote the formation of η phase and cause embrittlement, so the Si content should be 0. It was set at 7 to 1.2%.

Like Si, Mn is an element necessary as a deoxidizing agent and is an element that combines with S to form MnS and prevents high-temperature cracking. However, when a large amount of Mn is added, it forms an η phase and causes brittleness. Therefore, the Mn content was set to 0.7 to 1.2%.

Mo is an important element in the present invention, and it strengthens the solid solution base without deteriorating high-temperature corrosion resistance against coal combustion gas, and some of it precipitates as carbide, improving high-temperature strength and forming crystals. Strengthen grain boundaries. Furthermore, it has the effect of suppressing the agglomeration and coarsening of carbides and γ' phase, which are strengthening factors, at grain boundaries. However, Mol is 0.8
If the content is less than 1.0%, the effect will be small, and if it exceeds 1.6%, the precipitation of the η phase will be promoted and the workability will be significantly reduced.

Nb, like Sr, Mn, Mol, or Ti, is an element that promotes the formation of the η phase, and is effective in precipitating carbides and improving high-temperature strength.

In addition, it forms fine and stable carbides and prevents the coarsening of crystal grains during use at high temperatures.It also suppresses the movement of C dissolved in the base of the r solid solution, which allows carbides to move to grain boundaries. Suppresses agglomeration and coarsening.

However, if the Nb content is less than 0.6%, the effect will be small, and if it exceeds 1.5%, η phase will be generated and the creep rupture strength will be reduced.
%.

AA is an element that is effective in improving oxidation resistance and also contributes to the appearance of the γ' phase, which is a feature of the present invention. Usually, N 13 (Al, T I
) Contributes to improving high-temperature strength by generating intermetallic compounds. However, when the amount of A4 is less than 0.3%, the amount of Ti contained in the γ' phase increases, and Ni3Ti, that is, the η phase, is precipitated and the strength is significantly reduced. Also 0, 7
%, excess Al will precipitate AlN, significantly reducing ductility.

Like Nb and Mo, Ti forms carbides and strengthens by precipitation, improving high temperature strength and ductility, and when added in combination with Al, precipitates the γ' phase to improve high temperature strength. This effect works effectively when the Al/Ti ratio is in the range of 0.2 to 0.5, and when it is less than 0.2, the high temperature strength is significantly lowered by precipitation of the η phase. Moreover, if it exceeds 0.5, the effect becomes small. Al amount and (Al/T
i) From the relationship of the ratio, the amount of Ti is 0.6 to 2.0%.

Particularly preferred is 0.9 to 1.6%.

Fe increases hot workability and strength, or Nb1Ti,
It is indispensable for adding alloying elements such as Mo using a 7-E alloy, but it is preferably 10% or less. In particular, 2 to 6% is preferable. (Al/Ti
) ratio is 0.2 to 0.5, which gives the highest strength and excellent corrosion resistance. 0.25 has better corrosion resistance.
~0.45.

The alloy of the present invention has an all-austenitic structure as solution treated. Solution treatment temperature is 1,100 to 1,20
0C is preferred, and 1150-1175G is particularly preferred.

[Embodiments of the invention]

Inventive steels and comparative steels having the chemical compositions shown in Table 1 were melted and then ingot-rolled into steel plates, which were subjected to solution treatment and then used as test materials.

The solution treatment was carried out on the invention steels A1 to 10 and the comparative steels All to
About 18 1175CX20 Seki water cooling.

Comparative copper metal 19.20 about 1150CX30m water cooling,
Comparative steel boiler 21 was water-cooled with 1050CX30.

Next, a corrosion test and a creep rupture test were performed on each sample material subjected to the solution treatment, and the corrosion resistance and creep rupture strength were compared.

In addition, the corrosion test was performed using a molar ratio of Naz804:KzSO4:
A mixture of FezO1 with a ratio of 1.5:1.5 + 1 was applied to the surface of the test material and heated to 1% 802 at 750C.
゜1 in a mixed gas of 10% CCh, 5% 02 and N2
The corrosion resistance was evaluated based on the corrosion weight loss after the test. In addition, '750 C, 10' hour creep rupture strength is 700 C, 750 C.

The creep rupture strength at 800C was evaluated using Larnon-Miller parameters.

The results are shown in Tables 2 and 3.

As shown in Table 2, 30% Cr of the present invention steel and comparative steel
-60%Ni-based material is comparative steel Todoroki 19 (SUS304)
) and A20 (Incoloy 800H), the corrosion resistance is about 6 times and 3 times higher, respectively, and the same corrosion resistance as the comparative steel A21 (Inconel 690).

FIG. 1 is a diagram showing the relationship between the weight ratio of (Al/T i ) and corrosion loss. The figure is a diagram connected for the Ti content of 0.9 to 1.6%. As shown in the figure, (Al
/Ti>When the ratio is 0.25 to 0.5, the amount of corrosion is small;
In particular, the most remarkable effect is seen when the ratio is 0.25 to 0.45.

As shown in Table 3, the 30%Qr-60%Ni alloy inland water invention steel with an (Al/Ti) ratio of 0.2 to 0.5 is 1.5 to 2 times that of the comparative steel. Has strength.

Tables 2 and 3 are diagrams showing the relationship between the (Al/Ti) ratio and the 750CXi, 000h clip rupture strength.

As shown in the figure, the value of (Al/T i ) is between 0.2 and 0.
5 has the highest strength.

FIG. 3 shows the creep rupture strength of the invention steel (A7 steel), comparative steel (A14 steel) outside the Al/Ti range, and Inconel 690 (A21). Boiled 7 steel changes linearly and shows high strength, while Boiled 14 steel has higher strength compared to Inconel 690 of flat 21, but tends to decrease in a curved manner over a long period of time.

〔Effect of the invention〕

According to the present invention, a forgeable, high-strength eave/high-corrosion-resistant alloy that has excellent high-temperature corrosion resistance and high creep rupture strength even in high-temperature, high-pressure coal combustion gas and high-temperature steam can be obtained, and this alloy can be used in thermal power generation plants. By applying it to coal-fired boiler tubes, it is extremely effective in improving power generation efficiency and effectively utilizing coal.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the (Al/Ti) ratio and corrosion loss, Figure 2 is a diagram showing the relationship between the (Al/Ti) ratio and creep rupture strength, and Figure 3 is a diagram showing the relationship between the (Al/Ti) ratio and creep rupture strength. This is a diagram showing the relationship between creep rupture strength and rupture time.

Claims (1)

[Claims] 1. By weight, C: 0.03~0.15%, Ni: 55~
65%, Cr: 28-32%, Mn: 0.7-1.2%
, Si: 0.7-1.2%, Mo: 0.8-1.6%,
Nb: 0.6-1.5%, Al: 0.3-0.7%, T
i: 0.6 to 2.0% and the balance is Fe, (Al/
A high-strength, high-corrosion-resistant alloy characterized by having a weight ratio of Ti) of 0.2 to 0.5 and having an entirely austenitic structure. 2. In claim 1, 600 to 800°C
A high-strength, high-corrosion-resistant alloy that is used as a material for seamless boiler tubes in environments exposed to coal combustion gas and combustion ash. 3. By weight, C: 0.03~0.15%, Ni: 55~
65%, Cr: 28-32%, Mn: 0.5-1.5%
, Si: 0.5-1.5%, Mo: 0.8-1.6%,
Nb: 0.6-1.5%, Al: 0.3-0.7%, T
i: 0.6 to 2.0%, the total of one or more of Zr, Hf, and Ta 0.03 to 0.5%, and the balance is Fe, and the weight ratio of (Al/Ti) is 0. .2 to 0.5, and has an entirely austenitic structure.