JPH0672286B2 - ▲ High ▼ Austenitic stainless steel with excellent temperature strength - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an austenitic steel excellent in high temperature strength, and more particularly to an austenitic stainless steel suitable for use in a casing for ultra-supercritical steam turbines or a valve body.

[Background of the Invention]

In order to cope with the depletion of oil and the soaring prices, it is considered to improve the efficiency by increasing the temperature and pressure of power plants. The steam power plant is currently operated under the steam condition of 538 ° C, and its casing and valve body materials are Cr-Mo-V.
In addition, ferritic heat resistant steel such as 12Cr is used.

However, these ferrite heat-resistant steels have a drawback that the grain boundary slip becomes remarkable in the temperature range of 550 ° C or higher and the creep strength is extremely lowered, and it is difficult to use them under steam conditions of 600 ° C or higher.

Austenitic heat-resistant steels currently used in the temperature range of 600 ° C and above include SUS304 and SUS314 steels. The creep rupture strength of these materials at 650 ° C for 10 ⁵ hours is the creep data sheet No. .4A (197
8) and the high-temperature strength data collection for metal materials (1975), one example is described, it is about 7 kg · f / mm ² .
The creep rupture strength required for the casing for ultra-supercritical steam turbines or the valve body is 8.5 kg as a design required value.
・ F / mm ² or more. Therefore, conventional austenitic heat resistant steels such as SUS304 and SUS316 steels do not meet the design requirement.

On the other hand, in order to improve the high temperature strength of the existing austenitic heat resistant steel, there is an example in which carbide forming elements such as Nb, Zr and Ti are added alone. However, since these elements are more stable in the nitride than in the carbide, they easily form NbN, TiN, ZrN and the like. Moreover, since these nitrides have almost no solubility in the matrix and form large horn crystals and are distributed in the grain boundaries within the grains, it is considered necessary for precipitation hardening of the alloy. Therefore, although the austenitic heat-resisting steels strengthened by adding Nb, Zr, Ti, etc. satisfy the creep rupture strength in a short time, the creep rupture strength in the long time side where N is absorbed from the atmosphere is increased. Is significantly reduced.

Furthermore, Cr ₂ N precipitates at grain boundaries near the cracks, which adversely affects the fatigue life of crack propagation from the surface. Therefore, in addition to creep, it is necessary to particularly prevent the generation of nitrides in the steam turbine casing and the valve body material that causes thermal fatigue due to start and stop. In terms of design requirements for casings for ultra-supercritical steam turbines, In
High-Ni heat-resisting steels such as coly800,15-15N, G13B satisfy the design requirements, but it seems difficult to melt large steel ingots for casing materials in terms of manufacturing technology.

[Object of the Invention]

An object of the present invention is to provide an austenitic stainless steel that exhibits sufficient creep rupture strength as a casing for 600 to 700 ° C. super-supercritical steam turbines or a valve body, and that is easy to manufacture and excellent in high-temperature strength.

[Outline of Invention]

The present invention suppresses the precipitation of harmful nitrides such as BN, NbN, TiN and Cr ₂ N by the addition of Al, which has a high affinity for nitrogen, and
The mother phase is strengthened by the interaction with Mo, and the stable carbide forming element or B is added to improve the creep rupture strength.

Hereinafter, the reasons for limiting the alloy composition in the present invention will be described. In addition,% described below is% by weight.

Carbon combines with the carbide-forming element to precipitate as carbide,
Improves creep rupture strength. However, if added in a large amount, the toughness and weldability are significantly reduced, so the upper limit is made 0.15%.

Silicon is an important deoxidizing component in production. However, if added in a large amount, it adversely affects toughness, ductility and weldability, so the upper limit is made 1.0%.

Manganese is an important deoxidizing component like silicon. However, the smaller the amount, the better the oxidation resistance, so the upper limit is made 2.0%.

Nickel (Ni) is an important element that forms an austenite structure. If the amount is small, this effect is small and the lower limit is 1
0.0% On the other hand, nickel acts on chromium to improve the corrosion resistance, but if it is a large amount, it becomes expensive, so its upper limit is set.
20.0%

Particularly, 8 to 18% is preferable.

Chromium (Cr) is an important additive element for improving the oxidation resistance, and its effect is low in a small amount, so the lower limit is 13.0%. However, if contained in a large amount, not only the weldability is deteriorated, but also adverse effects such as the formation of δ-ferrite, which promotes embrittlement during high-temperature use, are set, so the upper limit is made 25.0%.

Particularly, 13 to 17.13% is preferable.

Molybdenum (Mo) strengthens the material by forming a solid solution in the matrix, and improves creep rupture strength by combining with carbon to precipitate carbides. If less than 1.0%, the effect is low,
Also, if contained in a large amount, the workability deteriorates, so the upper limit is set.
2.6%

Aluminum (Al) is a deoxidizer for the molten metal and strengthens the matrix phase by interacting with molybdenum. Furthermore, since it has a high affinity with nitrogen and suppresses the precipitation of harmful nitrides such as NbN, TiN, Cr ₂ NBN, it is a particularly important additive element for improving creep rupture strength, but its effect is not exhibited at 0.01% or less. , 0.1% or more, the cleanliness in steel is impaired and the creep rupture strength decreases. Therefore, aluminum is 0.01
The range is limited to 0.1%.

Boron (B) is an important element for improving creep rupture strength, especially long-term creep strength. The effect is Ti
By making the amount 0.18% or more, it becomes remarkable at 0.002% or more. On the other hand, if the Ti content is 0.16% or more, it becomes remarkable at 0.068% or more, and if it exceeds 0.012%, the strength is decreased, so
The upper limit is 0.012%. Further, it is desirable to add B in a range satisfying P + S≤0.088-2.82B because the weld hot cracking susceptibility can be lowered.

Titanium (Ti), like molybdenum, forms a solid solution in the matrix to strengthen the material and precipitates carbides to improve creep rupture strength. However, when the B content is 0.002 to 0.0068%, the Ti content is
When the Ti content is less than 0.18% and the B content is 0.0068 to 0.012%, the effect is small when the Ti content is less than 0.16%, and conversely, when the Ti content is more than 0.3%, the strength is rapidly lowered, so that the content is dependent on the B content.

Copper can be contained up to 4%. Ni8 when containing copper
It is preferably set to -18%.

Each of the above elements is an essential component in the present invention, and each of the following elements is an optional component, and any two or more of them are added to the alloy.

Niobium (Nb) forms stable carbides and improves creep rupture strength. While this effect appears at 0.05% or more, addition of a large amount slightly lowers the oxidation resistance. Therefore, the upper limit is 0.4%. It is preferably 0.15% or less. Nb contains a small amount of Ta.

Vanadium (V) is an effective element for improving the corrosion resistance, and if it is less than 0.1%, its effect does not appear.
%, The balance of austenite becomes unstable, so the range was set to 0.1-0.5%.

Zirconium (Zr) is an element that refines crystal grains to improve high-temperature strength and ductility, but if added in a large amount it affects manufacturability, so an upper limit of 0.3% can be added. Calcium (Ca) increases castability. Can be added for. The proper amount is 0.003 to 0.1%, and grain boundary eutectic inclusions are eliminated.

The rare earth element can be added to improve mechanical properties and stabilize austenite and hardenability, but the effect is remarkable at 0.25% or less.

Example of Invention

 Examples of the present invention will be described below.

<Example 1> FIG. 1 is a cross-sectional view showing an example of a steam turbine using a casing of the austenitic stainless steel of the present invention.

In FIG. 1, a rotor 12 in which a plurality of moving blades 10 are planted is
It penetrates through an inner casing 16 provided with a plurality of vanes 14 so as to be located between the rotor blades 10. The inner casing 16 is formed with a plurality of convex portions 18, and the plurality of convex portions 18 are formed.
The outer casing 20 having the inner casing therein is fitted in the recess and is fixed by bolts or the like. Further, the outer casing 20 rotatably supports both ends of the rotor 12 in the through holes 22, and has an outlet 24 formed in the lower left portion and an opening 26 formed in the upper portion in the figure.

The main steam flows down in the main steam pipe 30 as shown by the arrow, and flows into the inner casing 16 via the nozzle box 28. After that, when the rotor blade 10 is rotated integrally with the rotor 12,
The main steam enters the space between the inner casing 16 and the outer casing 20 and flows out from the outflow port 24.

Here, assuming that the temperature of the main steam is 650 ° C. and the pressure is 350 kg / f / cm ² , the steam turbine has a temperature of 650 ° C. to 554.3 ° C. and a pressure of 350 kg / f ^{2 to} 199 kg / f / c in the inner casing 16.
m ^2, a temperature 554.3 ° C. in the outer casing, pressure 199Kg ·
f / cm ^2, to obtain a maximum pressure difference 151kg · f / cm ² of the inner and outer casings.

As a material used for the inner casing under such operating conditions, the required design strength as shown in Table 1 is required.

The material of the present invention is used as an austenitic stainless steel used for the inner casing of such a steam turbine.
For comparison, T5, T7, T8 and T12 and general austenitic heat-resistant steel were smelted for comparison materials T1 to 4, T6, T9 to 11 (U1 to 6, S1
~ 3) and the creep rupture strength was compared. The creep rupture test was performed at a temperature of 650 ° C. and a stress of 17.5 kg · f / mm ² .

The chemical compositions of the invented material of the present invention and the comparative material are as shown in Table 2. The P amount in this embodiment is 0.012 to 0.031%.
And the amount of S is 0.003-0.012%. All of them were castings, and were subjected to solution treatment by heating at 1,100 ° C. for 1 hour and then water cooling.

FIG. 2 is a diagram showing the results of the creep rupture test.
The relationship between the amounts of Ti and B added to austenitic stainless steel and the breaking time is shown.

As is clear from the figure, it is necessary to meet the design requirement value of 17.5 kgf / mm ² under the creep condition of 650 ° C × 10 ³ h, but in order to achieve higher strength, B0.002 to less than 0.0068% and Ti0.18 ~
It should be 0.3% or B 0.0068-0.012% and Ti 0.16-0.3%.

<Example 2> Next, as shown in Table 3, B was added to the austenitic heat resistant steel.
The effect on hot cracking susceptibility during welding was investigated by changing the added amount.

The hot cracking susceptibility evaluation method for welds is the Barlestrain test, which is the material to be welded (plate thickness 10 mm, plate width 40 mm, length 300 mm).
It is a method of welding in the longitudinal direction on the upper surface of TiN by the TiG welding method, and sharply bending it to a strain amount of 0.5% during welding. The welding conditions are welding voltage of 10.5 V, current of 110 A and speed of 180 mm / mim. The hot cracking susceptibility was evaluated by counting only the hot cracks within a range of about 20 mm from the welded portion after the test as the number of cracks generated.

FIG. 3 is a diagram showing the relationship between the amount of B added to austenitic stainless steel and the number of cracks at high temperature. Hot cracking is P +
When the amount of S is around 0.04%, the amount of B is around 0.015%, and when the amount is more than 0.015%, it occurs. Therefore, from the viewpoint of preventing weld cracking, it is clear that the B content is preferably 0.015% or less.

FIG. 4 is a diagram showing the relationship between B and P + S, which was obtained by performing a Balestrain test using the test materials shown in Table 3. P and S are impurity elements that promote the hot crack susceptibility of the welded portion, and by lowering them, the hot crack susceptibility can be lowered. According to the figure, by lowering P + S,
The B content, which enhances the high temperature strength, can be increased. That is, in the relationship between B and P + S, P + S ≦ 0.088−2.8
If it is a region that satisfies the formula of 2B, B is
It was revealed that the content could be up to 0.03%.

FIG. 5 shows the creep rupture strength of the material of the present invention (T7: white circle) and the comparative material (average value of S1 to S3: dotted line). Compared with the comparative material, the inventive material has creep rupture strengths of 18 kgf / mm ² (improved by about 45% compared to the comparative material) and 9.2 kgf / mm by extrapolation at 10 ³ h and 10 ⁵ h, respectively. ² (up about 30% compared to the comparative steel), which shows that the design requirement values shown in Table 1 are satisfied.

Further, FIG. 5 also shows the results of the examination of the welded joint strength of the present invention. As the base material, the T7 material shown in Table 2 was used. The shape of the welded joint test piece has a plate thickness of 50 mm and a plate width of 15
The length is 0 mm and the length is 300 mm, and a U-shaped groove with a width of 30 mm is provided in the length direction. A commercially available AWS E316-16 coated arc welding rod was used as the welding rod. Welding is performed at a temperature between passes of 150 ° C or less. Joint welding was performed by the above method to produce a joint welding creep test piece (parallel portion 10φ). The black circles in the figure show the joint welding test results. As a result, the joint strength of the material of the present invention is the same as that of the base material, and it is clear that the welding performance of the material of the present invention is excellent.

Based on the above, austenitic stainless steel is coated with Al, B, Ti
By adding V, Ca, Zr, and rare earth elements in a small amount by adding a complex amount of V, Ca, Zr and rare earth elements, it has a creep rupture strength of 10 ³ h, which is about 45% higher than conventional austenitic stainless steel, and a casing for ultra-supercritical turbines. Also satisfies the design requirement value of.

〔The invention's effect〕

As described above, according to the present invention, it is possible to provide an austenitic stainless steel that has high creep rupture strength and is effective as a material for a casing for ultra-supercritical pressure turbines or a valve body.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of a steam turbine using the austenitic stainless steel of the present invention for a casing, FIG.
The figure shows (a) Ti and FIG. 2 (b) is a diagram showing the relationship between the added amount of B and the creep rupture time, and FIG. 3 shows the added amount of B in austenitic stainless steel and the number of cracks at high temperature. A diagram showing the relationship, Fig. 4 shows B and P in the Balestrain test.
FIG. 5 is a diagram showing the relationship with + S, and FIG. 5 is a diagram showing the creep rupture strength of the material of the present invention and the comparative material. 10 ... moving blade, 12 ... rotor, 16 ... inner casing, 20 ... outer casing.

 ─────────────────────────────────────────────────── --- Continuation of front page (72) Masayuki Yukawa, Inventor Masayuki Yukawa, 3-1-1, Saiwaicho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Masao Shiga, 3-chome, Saiwaicho, Hitachi, Ibaraki No. 1 Inside Hitachi Research Laboratory, Hitachi Ltd. (72) Inventor ▲ Yoshi ▼ Takatoshi Oka 3-1-1, Saiwaicho, Hitachi City, Ibaraki Inside Hitachi Research Institute Hitachi Ltd. (72) Inventor Katsumi Iijima 3-1, 1-1 Saiwaicho, Hitachi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Norio Yamada 3-1-1, Saichocho Hitachi City, Ibaraki Hitachi Ltd. Hitachi Research Institute (72) Inventor Yoshimitsu Tobita, 3-1-1, Saiwaicho, Hitachi, Ibaraki, Ltd., Hitachi Research Laboratory, Hitachi, Ltd. (56) Reference JP-A 59-70752 (JP, A) JP-A 59-100219 JP, A)

Claims (4)

[Claims] 1. A weight ratio of C: 0.15% or less, Si: 1.0% or less, Mn
2.0% or less, Ni: 10.0 to 20.0%, Cr13.0 to 25.0%, Mo: 1.0 to
2.6%, Al: 0.01 to 0.1%, Nb: 0.05 to 0.4%, B: 0.002 to 0.
Less than 0068% and Ti: 0.18-0.3% or B: 0.0068-0.012%
And Ti: 0.16 to 0.3%, with the balance being essentially Fe, an austenitic stainless steel excellent in high temperature strength. 2. In the claim 1, the main steam temperature is 600 to 700 ° C., the pressure is 250 to 400 kg · f / cm ² , and the high temperature strength is high. Excellent austenitic stainless steel. 3. A weight ratio of C: 0.15% or less, Si: 1.0% or less, Mn
2.0% or less, Ni: 10.0 to 20.0%, Cr13.0 to 25.0%, Mo: 1.0 to
2.6%, Al: 0.01 to 0.1%, Nb: 0.05 to 0.4%, Cu: 4% or less,
B: 0.002-0.0068% and Ti: 0.18-0.3% or B: 0.006
An austenitic stainless steel excellent in high-temperature strength, characterized by containing 8 to 0.012% and Ti: 0.16 to 0.3%, and the balance being substantially Fe. 4. A weight ratio of C: 0.15% or less, Si: 1.0% or less, Mn
2.0% or less, Ni: 10.0 to 20.0%, Cr13.0 to 25.0%, Mo: 1.0 to
2.6%, Al: 0.01 to 0.1%, Nb: 0.05 to 0.4%, V: 0.1 to 0.5%
And B: 0.002 to less than 0.0068% and Ti: 0.18 to 0.3% or B:
An austenitic stainless steel excellent in high temperature strength, characterized in that it contains 0.0068 to 0.012% and Ti: 0.16 to 0.3%, and the balance is substantially Fe.