JPS6223949A - High strength alloy having high corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a high strength Ni alloy having high corrosion resistance and superior creep rupture strength and suitable for use as a material for a boiler for steam-power generation using coal as fuel by adding specified amounts of Cr and other elements to Ni. CONSTITUTION:An Ni alloy ingot consisting of, by weight, 0.03-0.15% C, 28-32% Cr, 0.2-0.8% Mn, 0.7-1.2% Si, 0.8-1.6% Mo, 0.6-1.5% Nb, 0.3-0.7% Al, 0.6-2.0% Ti and the balance >=58% Ni combined optionally with Fe or further contg. 0.03-0.5% one or more among Zr, Hf and Ta and having 0.3-0.7 weight ratio of Mn/Ti is rolled. The resulting plate is subjected to soln. heat treatment by heating at 1,185 deg.C for 10min and water cooling to obtain a high strength alloy having high corrosion resistance and >=11kgf/mm<2> creep rupture strength after the lapse of 10<5>hr at 700 deg.C. The alloy has 100-350mum grain size and is used as a material for a boiler for steam-power generation using coal as fuel and operated at 600-700 deg.C high temp. under high pressure.

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, particularly when the steam temperature is 600 to 700°C in steam boilers for thermal power generation using coal as fuel. This invention relates to a high-strength, high-corrosion-resistant alloy suitable for use at high temperatures and high pressures.

[Background of the invention]

Conventional coal combustion H? Irate tubes, especially superheater tubes, are materials that are used in a severely corrosive environment due to combustion gas and attached ash (mainly alkali salts), which can lead to failure due to the synergistic effects of corrosion on the outer wall of the tube, creep, and fatigue damage. Often. The main causes of this corrosion thinning are SO2 contained in the combustion gas and low melting point Na2 contained in the combustion ash.
It is thought that this is due to the fact that 804 and 2804 attack the outer wall of the tube. On the other hand, recent thermal power plants have been strongly promoted to shift from conventional heavy oil combustion to coal combustion and to increase the temperature and pressure of steam conditions in order to conserve resources, conserve energy, and improve power generation efficiency. Not only is the corrosive environment becoming more severe, but materials with higher high-temperature strength than before are required.

By the way, in conventional coal-fired boilers, the maximum tube outer wall temperature was around 600°C and the steam pressure was around 246 η12, so SU830 Q series stainless steel, especially SU
S 316, SUS 321, SUS 347
etc. were used. In these plants, the main causes were thinning due to corrosion and steam oxidation during long-term use, which was combined with damage due to creep and fatigue, leading to destruction. Conventional countermeasures against corrosion caused by combustion gas and adhered ash include applying Chromisong treatment to the outer wall of the tube, adding rare earth elements to form a dense and stable film with good adhesion, and using low-melting-point alkali salts. This is being dealt with by increasing the melting point of the substance.

In addition, as a countermeasure against steam oxidation on the inner wall of the tube, shot peening or the like is used to make the crystal grains on the steam contact surface finer.

However, when looking at, for example, chromisong treatment and peening, 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, an alloy with high Ni and Cr contents is required that has both high temperature strength and corrosion resistance.

As such an alloy, Sup@rt is an iron-based alloy.
herm (27Cr-36Ni-15Co), Incoloy 800 (21Cr-36Ni-0,4Al-0
, 4TI)%, but the former has problems in processing and welding into tubes, and the latter has slightly insufficient strength. In addition, many 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 manufacture seamless pipes by drawing. Alloys cannot be welded. Therefore, as a high-strength, high-corrosion-resistant alloy, it has a high Cr content and a stable austenitic structure even after long-term heating at high temperatures. material is required.

As a high Cr-N1 alloy having strength and corrosion resistance, for example, as shown in JP-A-52-92818, Cr is 10 to 40% by weight and Al is 0.5 to 5% by weight.
%, Ni, (A
There are alloys that obtain strength by aging precipitation of the r' phase, but this kind of r' precipitation-strengthened alloy has problems in the construction process as described above, so it cannot be used as a flat tube. It's inappropriate. Also, JP-A-50-124
"N1 with excellent heat resistance and corrosion resistance" shown in Publication No. 822
-Cr alloy' has a high Ti and HT content of 1.8 to 2.8 Ti, and also C.

A Ni-based alloy containing 16 to 24 pieces of Ni, like the former, is an age-precipitation-strengthened alloy and is unsuitable for use as a silica tube. The "chromium-nickel alloy" disclosed in Japanese Patent Application Laid-open No. 47-22823 contains 40 to 55% of Crf, Nb, and T.
a, Ti t-added Ni-based alloy.This alloy has nitride-forming elements added to suppress the decrease in ductility at room temperature after long-term heating at high temperature. Irachu! It is difficult to apply C to strength members such as . Furthermore, as shown in Japanese Patent Application Laid-open No. 52-68021, "A heat-resistant alloy containing 24 to 53% of old iNI and 20 to 44% of Cr, used in petrochemical plants as centrifugally cast pipes with higher C and 81. It is used for pipes, etc.
It is difficult to apply to 1′ ira tubes.

As mentioned above, in terms of strengthening factors, 1%Cr-N1 alloys are mainly centrifugal cast alloys with high ci or 7'''' precipitation type alloys with high Al and TI additions, so tubes can be There are problems with manufacturing, workability, welding, etc., making it difficult to use as a material for boiler tubes. Therefore,
Currently, Inconel 690 is optimal as a high-Or Geirachuso material, but it has the problem of low high-temperature strength, particularly low creep rupture strength.

[Purpose of the invention]

An object of the present invention is to provide a high-strength, high-corrosion-resistant alloy that has both creep rupture strength and corrosion resistance, and is particularly suitable for use in coal-fired thermal power generation boilers.

[Summary of the invention]

The characteristics of the alloy of the present invention that achieve the above-mentioned objects are: C: 0.03 to O, 15q6, Cr: 28 to 32% in weight ratio
, Mn: 0.2-0.8%, 81: 07-1.2
%.

Mo: 0.8-1.6 q6, Nb: 0.6-1.5
%, A/=: 0.3-0.7%, Ti: 0.6-2.0
%, the balance is at least 5896 N1 or more, or in some cases contains Fe together with a tooth, and is composed of impurities that are unavoidably mixed in during manufacturing, and has an entirely austenite structure. The alloy of the present invention further contains Zr, which is an element known to have the effect of suppressing the agglomeration and coarsening of carbides after high temperature and long-term heating, within the above range.

At least one or more of Hf and Ta can be contained as necessary. Zr, Hf, and Ta strengthen grain boundaries, precipitate fine carbides, and reduce M2 at grain boundaries.
Prevents strength reduction and crystal grain coarsening due to agglomeration and coarsening of 3C6 type carbides. When this is contained, the content is usually 0.0 as necessary and sufficient to obtain the above-mentioned suppressing effect.
3-0.5%, good 11. (It is preferable to set it in the range of 5 to 0.1%.

The reason why the content of each alloying element in the present invention is set as described above is as follows.

C is an austenite-forming element, and is effective in maintaining high-temperature strength by being dissolved in the matrix. Also, M
C, Nb, Tf, etc., form carbides and precipitate within the grains, improving high-temperature strength. However, if the C content is less than 0.03%, this effect decreases 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, if it exceeds 0.15%, carbides will precipitate at grain boundaries, aggregate and coarsen, resulting in a decrease in high-temperature strength and impairing workability and weldability.
Since it is fixed as r Z s Cb type carbide, the amount of solid solution Cr decreases and corrosion progresses preferentially from the Cr-deficient phase near the grain boundaries. That's all, C.
The content is between 0.03 and 0.15, especially between 0.04 and 0.
A range of 0.08% is preferred.

Ni is an element that coexists with Cr to improve plastic workability and stabilize the austenite structure, and maintains high-temperature strength by being solid-solved in the matrix, creating a complete austenite structure.
According to the Schaeffler diagram calculated from the relational expression between Nt equivalent and Cr equivalent, approximately 28% or more is required to obtain &, but in Cr-Ni or Fe-Cr-Ni alloys, Cr As the amount increases, σ in the range of 600-800℃
7, elongation, drawing strength and creep strength decrease (two-phase alloy of austenite and ferrite (r1α))
Or even if it is completely austenite (γ), a σ phase is generated).

This suppression of σ phase generation is related to the ratio of Ni to Cr.

For example, in an alloy containing about 30% of Cr, in order to have a stable austenitic structure, an amount of Ni of about twice the phase degree is required.

In addition, in the present invention, by adding Ar and Ti in a composite manner, Ni 3 (Al, Ti) intermetallic compound, that is, r' phase, is precipitated during use at high temperatures, thereby improving high temperature strength, especially creep rupture strength. However, the amount of added Al and Ti affects the generation of r', and the amount of Ni also plays an important role. Also, the cr amount is 3
When the Ni system is kept constant at 0% and the Ni content is varied from 20 to 7 degrees, the Ni i has a large effect of improving corrosion resistance.

Furthermore, the present invention has a characteristic configuration in which the content of Mn, which is considered an austenite stabilizing element in suppressing the formation of the η phase, is lowered as described below in consideration of the workability of the alloy. .

Therefore, due to the need to increase the amount of Ni in various ways, N
The content of i is at least 58% or more, especially 60 to 67q
A range of 6 is preferred.

F6 can be contained in place of a part of Ni, but in this case as well, at least 58% or more of Ni is required, so it is particularly preferable that F. is 100% by weight or less, especially 5% by weight or less.

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 inches is required, and from the viewpoint of suppressing the formation of the η phase, it is not preferable to add more than 32 inches.

Therefore, the Cr content is 28 to 32 inches.

Si has a deoxidizing effect) and is an element necessary as a deoxidizing agent and an element that improves corrosion resistance. However, if too much Si is added, it will promote the formation of the η phase and cause embrittlement. is preferably in the range of 0.7 to 1.2%, particularly 0.7 to 1.0%.

Like Sl, Mn is necessary as a deoxidizing agent.
It is an element that combines with S, which is one of the elements that are inevitably mixed in during manufacturing, to form MnS and prevent high-temperature cracking.
Forged. When considering the production of seamless pipes and the like through rolling and drawing processes, if Mn is increased more than necessary, cracks are more likely to occur. Further, contrary to 81, Mn is an austenite stabilizing element and is responsible for high-temperature strength, so its content is preferably in the range of 0.2 to 08%, particularly 03 to 07%. In addition, TI is added to the material of the present invention in order to precipitate the γ' phase, that is, the N1a (Al, TI) intermetallic compound, which is an important strengthening factor. l), Mn/
Although the Ti ratio is not particularly limited, from the viewpoint of improving strength, the Mn/Ti flap ratio is preferably 0.3 to 0.7.

Mo is an important element in the present invention. Mo strengthens the solid solution base without deteriorating the high temperature corrosion resistance against coal combustion gas, and some of it precipitates as carbide to improve high temperature strength and strengthen crystal grains. strengthen the world. Furthermore, it has the effect of suppressing the agglomeration and coarsening of carbides and γ' phase, which are strengthening factors, at grain boundaries. However, when the amount of Mo is 0.8
If it is less than 1.6 mm, the effect is small, and if it exceeds 1.6 mm, the precipitation of the η phase will be promoted and the workability will be significantly reduced. Therefore, the Mo content is preferably in the range of 0.8 to 1.6%, particularly 1.0 to 1.3%.

Nb, like Si+Mn, Mo, 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, prevents coarsening of crystal grains during use at high temperatures, and suppresses the movement of C dissolved in the base of the r solid solution to the grain boundaries. Suppresses the agglomeration and coarsening of carbides. However, if Nb1l is less than 0.6 inches, this effect is small, and if it exceeds the 1.5 coefficient, η phase is produced and the creep rupture strength is reduced. Therefore, Nb
The content is preferably in the range of 0.6 to 1.5q6, particularly 0.9 to 1.3q.

AA is an element that is effective in improving oxidation resistance and also contributes to the precipitation of the γ' phase, which is a feature of the present invention. Usually, by combined addition with TI, N13/A/1. , T.I.)
Contributes to improved high-temperature strength by generating intermetallic compounds. However, when the amount of Al is less than 0.3 inches, the amount of Ti contained in the bi-phase increases and Ni3T+, that is, the η phase is precipitated, resulting in a significant decrease in strength.

Moreover, if it exceeds 0.7%, excess Al will precipitate AlN,
Significantly reduces ductility. Therefore, the Al content is 0
．． A range of 3 to 0.7%, particularly 0.3 to 0.5%, is preferred.

Like Nb and Mo, T1 forms carbides and strengthens by precipitation to improve high temperature strength and ductility, and when combined with Al, precipitates a ribbed phase to improve high temperature strength. This effect works effectively at an Al/T l ratio of 0.2 to 0,
If it is less than 0.2, the high temperature strength will be significantly lowered due to the precipitation of η phase.

Moreover, if it exceeds 0.5, the effect becomes small.

On the other hand, if it exceeds 2.0, the amount of precipitates increases and the ductility is significantly reduced. Therefore, the content of T1 is 0.6
~2. The OS is particularly preferably in the range of 0.9 to 1.6 inches.

The alloy of the present invention has an all-austenitic structure as solution treated. Solution treatment temperature is 1100-1200℃
is preferable, particularly 1170 to 1190°C. If solution treatment is performed in this range, the crystal grains will be approximately 100 to 350 μm.
The range is m. Grain size greatly affects creep rupture strength. Generally, if the steel is of the same type, the larger the crystal grains, the higher the strength; however, if the grains are too large, the short-term strength will be high, but the long-term strength will decrease. Therefore, it is preferable to obtain strength by limiting the crystal grains depending on the heat treatment temperature, and according to the solution treatment, 700℃,
io5 hour creep rupture strength is l 1 kgt7'■2
A material exhibiting a creep rupture strength of 7 kgf/mm or more at 750° C. or 750° C. for 10 hours can be obtained. Although there is no effect even if the final phase, which is a strengthening factor of the present invention material, precipitates in a volume ratio of about 0.5n during solution treatment, normally solution treatment is rapidly cooled by water cooling after heating. Late phase is not allowed.

[Embodiments of the invention]

The alloy of the present invention having the chemical composition shown in Table 1 (NO, 1 to NO.
5'), and comparative alloys (NO, 6 to No, 12)
was melted, ingot-formed and rolled into a steel plate, which was subjected to solution treatment and then used as a test material.

Solution treatment was carried out at 1185°C x 10rnln-water cooling for the present invention alloys NO, 1 to 5 and comparative alloys N016 to 9, and at 1000°C x 30 m1 for comparative alloys NO, 10.
n-Water cooling, 3150°C for comparative alloy NO, 11
30 m1n-water cooling, 10 for comparative alloy No. 12
Corrosion tests and creep rupture tests were conducted on the sample materials that had been subjected to water cooling at 50° C. x 30 ml and then subjected to solution treatment, and the corrosion resistance and crinib rupture strength were compared. Furthermore, the occurrence of cracks was investigated when a molten ingot with a diameter of 120 m and a length of 200 m was forged into a 3o square material, and the forgeability was compared.

In addition, the corrosion test was conducted using Na2SO4:2 at the -E-/l/ratio.
A mixture consisting of So4:Fe2O3 in a ratio of 1.5:1.5:1 was applied to the surface of the test material, and heated at 730°C for 11 so
2. It was maintained in a mixed gas of i0% CO2°5fiO2 and N2 for 100 hours, and the corrosion resistance was evaluated based on the corrosion loss after the test. It's a cabinet, 700℃, 750℃I 1
055 hour cleave strength is 700℃, 750'C,
From the creep rupture strength of 800℃, Larson's /
? The evaluation was made by organizing the results using parameters. Further, forgeability was evaluated by conducting actual forging as described above and investigating the occurrence of cracks.

The results are shown in Tables 2, 3 and 4.

As shown in Table 2, the 30-inch Cr-60-nickel alloy (referring to the sample material NO, 1-NO, 9; hereinafter referred to as the blank) is the comparative alloy NO, ] 0 (SU8310) and N
O.

11 (Incoloy 800H), respectively 6 times,
It exhibits about three times the corrosion resistance, and exhibits corrosion resistance equivalent to that of the comparative alloy NO,12 (Inconel 690).

Furthermore, as shown in Table 3, the Q-zo rupture strength of the alloy of the present invention was higher than that of the comparative alloy, with 30%Cr-80%
Comparative alloys NO, 6, and N008 also show a tendency for strength to decrease over a long period of time.

Table N4 shows the results of evaluating the forgeability of the 30q6Or-604Ni alloy based on the occurrence of cracks during forging. No cracking occurred in N 2 O, 6 and N 2 O, 8. However, the internal strength of the comparative alloys is 1. -1 Cracks occurred in NO47 and NO9.

Figure 1 shows the relationship between the 10' hour creep rupture strength at 700°C and 750°C and the Mn content, as shown in the results of Table 3 above. From this figure, it can be seen that the Mn content is 0.2 to
It shows high strength in the range of 0.8 inches, especially in the range of 0.3 to 0.
It is understood that 7% is preferred. Also, Figure 2 shows 700
°C and 750 °C (DIO-creep rupture strength and Mn/
'1''1 relationship is shown based on Tables 1 and 3. In this figure, j7 Mn/T1 is 0.3 to 0.
．． It is understood that high strength is shown in the range of 7. In addition, Fig. 3 shows the relationship between the Clinib breaking strength and the Larson-Miller) motherboard meter using the present invention alloy No. 5 and the comparative alloy No. 8 as examples. It is understood that the alloy has favorable properties such as higher strength than the comparative alloys and a gentle slope of the straight line.

Table 2 Table 3 Table 4 [Effects of the Invention] According to the present invention, for example, when used in an atmosphere of high temperature and high pressure coal combustion gas or high temperature steam, it has excellent high temperature corrosion resistance and high creep rupture strength. In addition, a high-strength, high-corrosion-resistant alloy suitable for manufacturing tubes, etc. by forging, rolling, and drawing can be obtained, and in particular, by applying this alloy to Ira tubes for coal-fired thermal power plants, it is possible to improve power generation efficiency1. It is extremely effective in effectively utilizing coal.

[Brief explanation of drawings]

Figure 1 shows the 105-hour creep rupture strength at 700°C and 750°C and the Mn fi
Figure 2 is a diagram showing the relationship between 105-hour creep rupture strength at 700°C and 750°C and Mn/T1, and Figure 3 is a diagram showing the relationship between creep rupture strength and Larson
FIG. 2 is a diagram showing the relationship between parameters of 2-color. Figure 1 Figure 2

Claims (7)

[Claims] (1) Weight ratio: C: 0.03 to 0.15%, Cr: 2
8-32%, Mn: 0.2-0.8%, Si: 0.7-
1.2%, Mo: 0.8-1.6%, Nb: 0.6-1
．． 5%, Al: 0.3-0.7%, Ti: 0.6-2.
1. A high-strength, high-corrosion-resistant alloy that can be forged and rolled, characterized in that the balance is essentially Ni, or Ni and Fe, containing at least 58% or more of Ni, and having an entirely austenitic structure. (2) A high-strength, high-corrosion-resistant alloy according to claim (1), having a Mn content of 0.3 to 0.7%. (3) In claim (1), Mn/Ti
A high-strength, high-corrosion-resistant alloy having a weight percent ratio of 0.3 to 0.7. (4) The high-strength, high-corrosion-resistant alloy according to claim (1), wherein the crystal grain size is 100 to 350 μm. (5) In claim (1), 700°C;
10^5 hour creep rupture strength is 11kgf/mm^2
High strength and high corrosion resistance alloy. (6) In claim (1), 750°C;
Creep rupture strength for 10^5 hours is 7kgf/mm^2
High strength and high corrosion resistance alloy. (7) Weight ratio: C: 0.03 to 0.15%, Cr: 2
8-32%, Mn: 0.2-0.8%, Si: 0.7-
1.2%, Mo: 0.8-1.6%, Nb: 0.6-1
．． 5%, Al: 0.3-0.7%, Ti: 0.6-2.
0%, the total of one or more of Zr, Hf and Ta is 0.03 to 0.5%, the remainder is substantially Ni or Ni
and Fe, containing at least 58% or more of Ni, and having an entirely austenitic structure.