JPH0660365B2 - High strength / high corrosion resistance alloy - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a heat-resistant alloy having improved high-temperature creep rupture strength, and particularly to a steam temperature of about 600 to 700 ° C. in a steam boiler for thermal power generation using coal as a fuel. It relates to a high-strength, high-corrosion-resistant alloy suitable for use in high temperature and high pressure.

[Background of the Invention]

Conventional coal-fired boiler tubes, especially materials used in a severe corrosive environment due to combustion and adhered ash (mainly alkali salts) such as superheater tubes, have the synergistic effect of tube thinning due to corrosion and creep and fatigue damage. It often leads to destruction. It is thought that the main cause of this corrosion thinning is that SO ₂ contained in the combustion gas and Na ₂ SO ₄ and K ₂ SO ₄ with low melting points contained in the combustion ash attack the outer wall of the tube. . On the other hand, in recent thermal power plants, the shift from conventional heavy oil combustion to coal combustion and high temperature / high pressure steam conditions are strongly promoted from the viewpoint of resource saving / energy saving and improvement of power generation efficiency. Not only the corrosive environment becomes more severe, but also a material having higher high temperature strength than ever is required.

By the way, in the conventional coal-fired boiler, the tube outer wall temperature was about 600 ° C. at maximum and the steam pressure was about 246 kg / cm ² , so SUS 300 series stainless steel, especially SUS 31
6, SUS321, SUS347, etc. were used. In these plants, mainly due to corrosion and steam oxidation due to corrosion during long-time use, damages due to creep and fatigue were superimposed on the above, leading to destruction. Conventionally, as a measure against the corrosion caused by the combustion gas and the adhered ash, the outer wall of the tube is subjected to a chromizing treatment, a rare earth element is added to form a dense stable film with good adhesion, and the melting point of a low-melting alkali salt is used. We are dealing with it by increasing. As a countermeasure against steam oxidation of the inner wall of the tube, shot peening or the like is performed to make crystal grains on the steam contact surface finer.

However, for example, regarding chromizing treatment and shot peening, there is a problem that the effect is reduced by the construction technique and welding when applied to an actual plant. Further, since the use environment becomes severe due to the high temperature and high pressure of steam conditions, it is necessary that the material itself has excellent high temperature gas corrosion resistance and steam oxidation resistance. Further, since the design strength with respect to the design dimension of the tube is increased by the high temperature and high pressure, the corrosion resistance and the high temperature strength are important factors. Therefore, an alloy having a high Ni and Cr content, which has both high temperature strength and corrosion resistance, is required.

Such an alloy includes Supertherm as an iron-based alloy.
(27Cr-36Ni-15Co), Incoloy 800 (21
Cr-36Ni-0.4Al-0.4Ti), etc., but the former has problems in tube processing and welding, and the latter has a slightly insufficient strength. In addition, many of the Ni-based alloys and Co-based alloys for gas turbine materials are cast steel,
Not only is it difficult to produce seamless tubes by drawing, but welding is not possible with age-hardened γ'precipitation strengthened alloys. Therefore, as a high-strength / high-corrosion resistant alloy, it has a high Cr content, has a stable austenite structure even after heating at high temperature for a long time, and has sufficient high-temperature strength up to solution treatment in consideration of drawing, bending, and weldability. Materials needed.

As a high Cr-Ni alloy having strength and corrosion resistance, for example, as disclosed in JP-A-52-92818,
A Ni-based alloy containing 10 to 40% by weight of Cr, 0.5 to 5% of Al, and 0.5 to 5% of Ti is Ni ₃ (Al, Ti), that is, γ ′.
There are alloys that gain strength by precipitating phases,
This type of γ'precipitation strengthened alloy is unsuitable as a boiler tube because of the problems in the construction process as described above. The "Ni-Cr alloy having excellent heat resistance and corrosion resistance" disclosed in Japanese Patent Laid-Open Publication No. 50-124822 has a high content of Ti and Al of 1.8 to 2.8% and a Co content of 16 to 24. %
It is a Ni-based alloy containing nickel, which is an age precipitation strengthening alloy as in the former, and is also unsuitable as a boiler tube. JP-A-47
"Chromium-nickel alloy" disclosed in JP-A-22823 is a Ni-based alloy containing 40 to 55% of Cr and added with Nb, Ta, and Ti. This is to suppress the decrease in ductility at room temperature after heating, and it is difficult to apply it as a strength member such as a boiler tube. Further, the "heat resistant alloy" disclosed in JP-A-52-68021 contains Ni of 24 to 53% and Cr of 20 to 44% and is used in a petrochemical factory as a centrifugal casting pipe having a higher C and Si. It is used for pipes, etc., and is difficult to apply to boiler tubes.

As described above, as a high Cr-Ni alloy, from the viewpoint of its strengthening factor, a centrifugally cast alloy with a high C content or high Al,
Since γ'precipitation-strengthened alloys containing Ti are mainly used, there are problems with tube manufacturing, workability, welding, etc., making it difficult to apply as a material for boiler tubes. Therefore, Inconel 690 is currently the most suitable as a high Cr boiler tube material, but it has a problem that the high temperature strength, especially the creep rupture strength, is low.

[Object 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 a coal-fired boiler for thermal power generation.

[Outline of Invention]

The characteristics of the alloy of the present invention for achieving the above-mentioned object are that, in the first invention, the weight ratio is C: 0.03 to 0.15% and Cr: 28 to 3
2%, Mn: 0.2 to 0.8%, Si: 0.7 to 1.2%, Mo:
0.8-1.6%, Nb: 0.6-1.5%, Al: 0.3-
0.7%, Ti: 0.6 to 2.0%, the balance being substantially Ni,
Or Ni and Fe, Ni is contained at least 58% or more, has a total austenite structure, and Mn / Ti
In the second invention, the weight ratio of C: 0.03 to 0.15%, Cr :: 28 to 32 is characterized by being 0.3 to 0.7. %, Mn: 0.2-
0.8%, Si: 0.7 to 1.2%, Mo: 0.8 to 1.6
%, Nb: 0.6 to 1.5%, Al: 0.3 to 0.7%, Ti:
0.6 to 2.0%, the total of one or more of Zr, Hf, and Ta is 0.03 to 0.5%, and the balance is substantially Ni, or Ni and Fe, and Ni is at least 58%. It is characterized in that it is contained as described above, has a total austenite structure, and has a Mn / Ti weight% ratio of 0.3 to 0.7.
Further, the alloy of the present invention further contains at least one or more of Zr, Hf, and Ta, which are known as elements having an effect of suppressing agglomeration and coarsening of carbides after heating at a high temperature for a long time, within the above range, if necessary. Can be included. Zr, Hf, and Ta strengthen the grain boundaries, precipitate fine carbides, and prevent the reduction of strength and the coarsening of crystal grains due to the coarsening of M ₂₃ C ₆ type carbides in the grain boundaries. When this is contained, the content necessary and sufficient for obtaining the above-mentioned suppression effect is usually 0.03 to 0.5%, preferably 0.05 to 0.1%.

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

C is an austenite forming element, and is effective in maintaining a high temperature strength by forming a solid solution in the matrix. Also, M
It forms carbides with o, Nb, Ti, etc. and precipitates in the grains to improve the high temperature strength, but if the C content is less than 0.03%, its effect is reduced and the σ phase is generated to embrittle, The creep rupture strength at high temperature for a long time also significantly decreases. Also,
If it exceeds 0.15%, carbide precipitates at the grain boundaries and coarsens agglomerates to lower the high temperature strength, and at the same time impairs workability and weldability. Furthermore, Cr, which is effective for improving corrosion resistance, is added to Cr ₂₃ C ₆ type. Since they are fixed as carbides, the amount of solid solution Cr decreases, and corrosion progresses preferentially from the Cr-deficient phase near the grain boundaries. From the above, the content of C is 0.03 to 0.
15%, especially 0.04 to 0.08% is preferable.

Ni is an element that coexists with Cr to enhance plastic workability, stabilizes the austenite structure, maintains a high temperature strength by forming a solid solution in the matrix, and Ni equivalent and Cr are required to obtain a complete austenite structure. According to the Siefler diagram calculated from the relational expression with the equivalent, about 28% or more is required, but in the case of Cr-Ni-based or Fe-Cr-Ni-based alloys, when the Cr content increases,
Σ phase is generated in the range of up to 800 ° C, and elongation, drawing and creep strength decrease (σ phase is generated even if it is a two-phase alloy of austenite and ferrite (γ + α) or perfect austenite (γ)). . The suppression of the formation of the σ phase is related to the amount ratio of Ni and Cr. For example, an alloy containing about 30% Cr needs about twice the amount of Ni in order to have a stable austenite structure.

Further, in the present invention, by adding Al and Ti in combination, a Ni ₃ (Al, Ti) intermetallic compound, that is, a γ ′ phase is precipitated during use at high temperature, thereby improving high temperature strength, particularly creep rupture strength. Although it has a characteristic structure, the addition amount of Al and Ti influences the formation of γ ′, and the Ni amount also plays an important role. Further, when the amount of Ni changes from 20 to 70% when the amount of Cr is kept constant at 30%, the greater the amount of Ni, the greater the effect of improving the corrosion resistance.

Furthermore, in the present invention, Mn, which is an austenite stabilizing element in terms of suppressing the formation of the σ phase, has a characteristic configuration in which it is lowered as described later in consideration of the workability of the alloy. .

Therefore, from the necessity of increasing the various amounts of Ni, the Ni content is preferably at least 58% or more, and particularly preferably in the range of 60 to 67%.

Fe can be contained by substituting a part of Ni,
In this case as well, Ni is required to be at least 58% or more, so that Fe is particularly preferably 10% by weight or less, particularly 5% by weight or less.

Cr is an element which is preferentially oxidized to form a protective film with good adhesion on the surface and is effective in remarkably improving the oxidation resistance. At least 28% or more is required to obtain sufficient oxidation resistance, and an addition amount exceeding 32% is not preferable from the viewpoint of suppressing the formation of the σ phase. Therefore, the content of Cr is 28 to 3
2%

Si has a deoxidizing action and is an element necessary as a deoxidizing agent and improving corrosion resistance. However, if too much is added, the formation of the σ phase is promoted and embrittlement occurs, so the Si content is 0. A range of 0.7 to 1.2%, particularly 0.7 to 1.0% is preferable.

Mn is an element necessary as a deoxidizing agent like Si, and is combined with S, which is one of the elements inevitably mixed in during manufacturing, to form MnS.
Is an element to prevent hot cracking, but when considering the production of seamless pipes by the steps of forging, rolling, and drawing, cracking tends to occur when Mn is increased more than necessary. In contrast to Si, Mn is an austenite stabilizing element and plays a role in high temperature strength, so its content is preferably 0.2 to 0.8%, and particularly preferably 0.3 to 0.7%. . Further, in the material of the present invention, Ti is added in order to precipitate the γ ′ phase which is an important strengthening factor thereof, that is, Nia (Al, Ti) intermetallic compound, and the Mn / Ti ratio is not particularly limited. However, from the viewpoint of improving strength, the Mn / Ti weight% ratio is preferably 0.3 to 0.7.

Mo is an important element in the present invention, and strengthens the base of γ solid solution without deteriorating the high temperature corrosion resistance to coal combustion gas, and part of it precipitates as carbide to improve high temperature strength and crystal grain Strengthen the world. Furthermore, it has the effect of suppressing the coarsening of the coagulation of the carbide and γ'phase, which are the strengthening factors, at the grain boundaries. However, if the amount of Mo is less than 0.8%, its effect is small, and if it exceeds 1.6%, the precipitation of the σ phase is rather promoted and the workability is remarkably lowered. Therefore, the Mo content is 0.8 to 1.6%, especially 1.0 to
A range of 1.3% is preferred.

Nb is an element that promotes the formation of the σ phase like Si, Mn, Mo, or Ti, but it is effective in precipitating carbides and improving the high temperature strength. Further, by forming fine and stable carbides to prevent the coarsening of the crystal grains during use at high temperature, and to suppress the migration of C dissolved in the base of the γ solid solution, the carbides to the grain boundaries can be reduced. Suppresses coagulation and coarsening. However, if the amount of Nb is less than 0.6%, its effect is small, and if it exceeds 1.5%, a σ phase is formed to lower the creep rupture strength. Therefore, the Nb content is 0.
A range of 6 to 1.5%, particularly 0.9 to 1.3% is preferable.

Al is an element that is effective in improving the oxidation resistance and contributes to the precipitation of the γ'phase, which is a feature of the present invention. Normal,
When added together with Ti, Ni ₃ (Al, Ti) intermetallic compound is generated, which contributes to improvement of high temperature strength. However, when the Al content is less than 0.3%, Ti contained in the γ ′ phase
The amount increases and Ni ₃ Ti, that is, the η phase is precipitated and the strength is remarkably reduced. On the other hand, if it exceeds 0.7%, excess Al becomes AlN.
To significantly reduce ductility. Therefore, the Al content is preferably 0.3 to 0.7%, particularly preferably 0.3 to 0.5%.

Similar to Nb and Mo, Ti forms carbides and strengthens the precipitation to improve high-temperature strength and ductility, and when combined 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 η phase is precipitated and the high temperature strength is significantly reduced. Further, if it exceeds 0.5, the effect becomes small. If it exceeds 2.0%, γ '
The amount of precipitates in the steel increases, and the ductility is significantly reduced. Therefore, the Ti content is 0.6 to 2.0%, especially 0.9 to 1.6.
% Range is preferred.

The alloy of the present invention has an austenitic structure as-solution treated. The solution treatment temperature is preferably 1100 to 1200 ° C, and particularly preferably 1170 to 1190 ° C. When the solution treatment is performed in this range, the crystal grains are in the range of about 100 to 350 μm.
The grain size has a great influence on the creep rupture strength.
Generally, in the case of the same steel type, the larger the crystal grains, the higher the strength. However, if the grain size is too large, the strength increases for a short time but decreases for a long time. Therefore, it is preferable to obtain the strength by limiting the crystal grains depending on the heat treatment temperature. According to the solution heat treatment, the creep rupture strength at 700 ° C for 10 ⁵ hours is 11 kgf / mm ² or more, or 750 ° C for 10 ⁵ hours. A creep rupture strength of 7 kgf / mm ² or more is obtained. Further, even if 0.5% by volume of the γ ′ phase, which is a strengthening factor of the material of the present invention, is not affected during the solution treatment, the solution treatment usually involves rapid cooling by heating and then water cooling. No γ'phase is observed after the chemical treatment.

Example of Invention

The alloy of the present invention having the chemical composition shown in Table 1 (NO. 1 to NO.
5) and the comparative alloys (NO. 6 to NO. 12) were ingot-ingot-rolled into steel plates, which were subjected to solution treatment and then used as test materials.

The liquefaction process is performed according to the alloy NO. 1-5 and comparative alloy N
O. Nos. 6 to 9 were 1185 ° C. × 10 min-water cooling, comparative alloy NO. About 10 1050 ℃ × 30 min-water cooling,
Comparative alloy NO. No. 11 1150 ° C. × 30 min-water cooling, comparative alloy NO. About 12 1050 ℃ × 30 min
-Water cooling respectively Next, the corrosion test and the creep rupture test were performed for each of the solution treated sample materials to compare the corrosion resistance and the creep rupture strength. Further, the occurrence of cracks when a molten ingot having a diameter of 120 mm and a length of 200 mm was forged into a 30 mm square bar was examined to compare forgeability.

The corrosion test was conducted by applying a mixture having a molar ratio of Na ₂ SO ₄ : K ₂ SO ₄ : Fe ₂ O ₃ of 1.5: 1.5: 1 to the surface of the test material at 730 ° C. It was kept for 100 hours in a mixed gas of 1% SO ₂ , 10% CO ₂ , 5% O ₂ and N ₂ , and the corrosion weight was evaluated after the test to evaluate the corrosion resistance. Also, 700 ° C, 750
° C., 10 ⁵ h creep rupture strength, 700 ° C., 750
Evaluation was performed by using the parameters of Larson-Miller arranged from the creep rupture strength at ℃ and 800 ℃. Also,
The forgeability was evaluated by investigating the occurrence of cracks by actually performing forging as described above.

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

As shown in Table 2, 30% Cr-60% Ni alloy (test material N
O. 1-NO. No. 9 of the comparative alloy). 10 (SUS310) and NO. No. 11 (Incoloy 800H) shows corrosion resistance about 6 times and about 3 times respectively, and the comparative alloy NO. It shows the same corrosion resistance as 12 (Inconel 690).

Further, as shown in Table 3, the creep rupture strength of the alloy of the present invention shows a higher value than that of the comparative alloy, that is, 30% Cr-60% Ni.
Comparison alloy NO. 6 and NO. No. 8 shows a tendency that the strength decreases on the long time side.

Table 4 shows the results of evaluation of the forgeability of the 30% Cr-60% Ni-based alloy by the occurrence of cracking during forging. The alloy of the present invention showed no cracking due to forging, and the alloy of the comparative alloy was NO. 6 and NO. No cracking occurred in No. 8 either. However, the NO.
No. 7 and NO 9 were cracked.

FIG. 1 shows the relationship between the 10 ⁵ hour creep rupture strength at 700 ° C. and 750 ° C. and the Mn amount shown in the results of Table 3 described above. 0.
High strength in 8% range, especially 0.3-0.7%
Is understood to be preferred. FIG. 2 shows the relationship between the 10 ⁵ hour creep rupture strength at 700 ° C. and 750 ° C. and Mn / Ti based on Tables 1 and 3.
From this figure, it is understood that Mn / Ti exhibits high strength in the range of 0.3 to 0.7. Incidentally, FIG. 3 shows the alloy N of the present invention.
O. 5 and comparative alloy NO. 8 shows the relationship between the creep rupture strength and the parameters of Larson-Miller. From this figure, the alloy of the present invention exhibits higher strength than the comparative alloy, and the preferable characteristic that the slope of the straight line is gentle. It is understood that it has.

[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 and high-temperature steam, it is excellent in high-temperature corrosion resistance and has high creep rupture strength, and by forging, rolling and drawing. High strength suitable for manufacturing tubes etc.
A highly corrosion-resistant alloy can be obtained, and particularly by applying this alloy to a coal-fired boiler tube for a thermal power plant, it is extremely effective in improving power generation efficiency and effectively utilizing coal.

[Brief description of drawings]

Figure 1 is a diagram showing the relationship of the present invention the alloy examples and 10 ⁵ hours creep rupture strength of 700 ° C. and 750 ° C. in Comparative Alloy Example and the amount of Mn, the second figure is the same 700 ° C. and 750 ° C. for ¹⁰⁵ hours Figure 3 shows the relationship between creep rupture strength and Mn / Ti.
It is a figure which shows the relationship between the same creep rupture strength and the parameter of Larsson-Miller.

Front page continuation (72) Inventor Masanobu Kirihara 4026 Kuji-cho, Hitachi City, Ibaraki Prefecture, Hitachi Research Laboratory, Hiritsu Manufacturing Co., Ltd. (56) Reference JP-A-60-100640 (JP, A)

Claims (6)

[Claims] 1. A weight ratio of C: 0.03 to 0.15% and Cr: 28 to 32.
%, Mn: 0.2 to 0.8%, Si: 0.7 to 1.2%, Mo: 0.8 to 1.6%, N
b: 0.6 to 1.5%, Al: 0.3 to 0.7%, Ti: 0.6 to 2.0%, the balance being substantially Ni, or Ni and Fe, with Ni being at least
A high-strength, high-corrosion-resistant alloy capable of forging and rolling, containing 58% or more, having a total austenite structure, and having a weight% ratio of Mn / Ti of 0.3 to 0.7. 2. A high-strength / high-corrosion-resistant alloy as defined in claim 1 having a Mn content of 0.3 to 0.7%. 3. A high-strength, high-corrosion-resistant alloy according to claim 1, wherein the size of crystal grains is 100 to 350 μm. 4. The method according to claim 1, wherein the temperature is 700 ° C.,
10 ⁵ h creep rupture strength of high strength and high corrosion resistance alloy which is 11 kgf / mm ² or more. 5. The method according to claim 1, wherein 750 ° C. and 1
High strength and high corrosion resistance alloy creep rupture strength of 0 ⁵ hours is at 7 kgf / mm ² or more. 6. A weight ratio of C: 0.03 to 0.15% and Cr: 28 to 32.
%, Mn: 0.2 to 0.8%, Si: 0.7 to 1.2%, Mo: 0.8 to 1.6%, N
b: 0.6 to 1.5%, Al: 0.3 to 0.7%, Ti: 0.6 to 2.0%, Zr, Hf,
And one or more of Ta in total of 0.03 to 0.5%,
The balance is substantially Ni or Ni and Fe, Ni is contained at least 58% or more, has a total austenite structure, and the weight% ratio of Mn / Ti is 0.3 to 0.7. Characteristically forged and rollable high strength and high corrosion resistant alloy.