JPH07197208A - High strength high chromium cast steel for high temperature pressure vessel - Google Patents

Abstract

PURPOSE:To increase high temp. strength of a high strength high chromium cast steel for high temp. pressure vessel having ductility and toughness, equal to those of conventional material, applicable to steam turbine for thermal power generation. CONSTITUTION:This cast steel has a composition consisting of, by weight ratio, 0.05-0.13% carbon, <=0.7% silicon, <=1% manganese, 9.5-12% chrominum, <=0.6% nickel, 0.1-0.3% vanadium, 0.01-0.2% niobium, 0.01-0.2% tantalum, 0.01-0.1% nitrogen, <=0.5% molybdenum, 0.9-3% tungsten, <=2.5% cobalt, and the balance other elements. Further, this cast steel is a high strength high chromium cast steel for high temp. pressure vessel, which holds the weldability of the conventional high chromium cast steel and is increased in strength at high temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength, high-chromium cast steel for use in high-temperature pressure vessels, which is a material used in places where high reliability is required, such as a vehicle interior and a valve compartment of a steam turbine for thermal power generation. .

[0002]

2. Description of the Related Art The steam turbine casing and valve chamber are exposed to high temperature steam and are subjected to high pressure at high temperature. Therefore, the materials used also have high temperature strength represented by creep rupture strength and proof stress. Is required. In addition, since the structure of these parts is complicated and thick, when high temperature steam flows in at the time of start-up, a compressive strain occurs in the locally heated part, so that a large tensile stress is applied to that part during cooling. May remain. Further, cracks due to thermal fatigue are likely to occur in these portions, and the presence of such cracks or defects may cause brittle fracture to develop into large cracks. In order to prevent this, it is necessary to increase the ductility and toughness of the material. In addition, when these parts are manufactured by casting, collection by welding is required, and welding is also required for assembling and working them. Therefore, good weldability is indispensable, and it is necessary to secure ductility and toughness in order to secure such weldability.

[0003] Conventionally, in the case where such a part subjected to such high temperature and high pressure is manufactured by casting, so-called 1% Cr-0.
5% Mo cast steel, 2.25% Cr-1% Mo cast steel and C
Low alloy cast steels such as r-Mo-V cast steel have been used. However, these materials do not always have sufficient high temperature strength, and also have a problem in oxidation resistance at high temperatures. Therefore, as a material excellent in high temperature strength and high temperature oxidation resistance, 8
So-called 12-chromium steel, which contains about 15% chromium, has been receiving attention.

12-chromium steel has a high chromium content of 8 to 15%, so that it has better oxidation resistance than the above-mentioned low alloy steel, and is excellent as a relatively small rolled material or forged steel material. Since it is widely used as a material with high temperature strength,
It is expected to have excellent high-temperature strength as a casting material. Many 12-chrome cast steels have already been developed, and JIS, A
Some are standardized in MS and ASTM. However, the composition of these materials has not been sufficiently investigated, so when manufacturing large castings and forgings such as steam turbine casings and valve chambers, the formation of δ ferrite due to segregation and the precipitation or quenching of coarse carbides Due to lack, sufficient ductility and toughness cannot be obtained. Further, even if ductility and toughness are obtained, high temperature strength is not obtained, so that practical use as a high temperature pressure vessel material has been difficult at present.

Therefore, the problem of reduction in ductility and toughness when a large material is used, which has been an obstacle to the practical use of conventional 12-chromium cast steel, is solved based on new tests and findings, and the creep rupture strength is also improved. An improved 12-chromium cast steel for high temperature pressure vessels has also been proposed [Japanese Patent Application No. 59-216].
No. 322 (Japanese Patent Laid-Open No. 61-96062). However, recently, from the standpoint of environmental protection such as prevention of global warming due to carbon dioxide, the efficiency of thermal power plants has been increased, specifically, the high temperature and high pressure of the main steam of steam turbines has been actively promoted. Therefore, such materials are not enough,
Development of cast steel for high temperature pressure vessels that can be used at higher temperatures is desired. That is, due to such a high temperature of the plant, there is a need for a material having a higher high-temperature strength while maintaining the weldability and oxidation resistance of the above-described 12 chrome cast steel for high-temperature pressure vessels.

[0006]

As described above, recently, the efficiency of thermal power plants has been improved, and the main steam of the steam turbine has been increased in temperature and pressure. There is a need for cast steel that has even higher high-temperature strength than the 12-chromium cast steel for containers. It is an object of the present invention to further improve high temperature strength while maintaining weldability and oxidation resistance equivalent to those already developed for a high temperature pressure vessel for 12 chrome cast steel.

[0007]

Therefore, as a result of intensive studies, the present invention has achieved the impact characteristics as a high temperature material with the weight ratios shown below, and the creep rupture strength is remarkably excellent. Invented steel chrome cast steel for high temperature pressure vessels.

That is, according to the present invention, (1) the weight ratio of carbon:
0.05-0.13%, silicon: 0.7% or less, manganese: 1% or less, chromium: 9.5-12%, nickel:
0.6% or less, vanadium: 0.1 to 0.3%, total of niobium and tantalum: 0.01 to 0.2%, nitrogen: 0.
01-0.1%, molybdenum: 0.5% or less, tungsten: 0.9-3%, cobalt: 2.5% or less, and unavoidable impurities and the balance being iron, the high temperature pressure vessel being characterized. To provide high strength and high chromium cast steel for use.

Further, the following cast steel in which the ratio of specific components among the components of the cast steel of (1) is changed is also provided.

(2) Cast steel in which the weight ratio of manganese in the above (1) is 0.1% or less.

(3) Cast steel in which the weight ratio of nickel is 0.2% or less in the above (1) or (2).

(4) A cast steel containing copper contained in impurities in the above (1), (2) or (3) in a weight ratio of 1.2% or less.

(5) In the above (1), (2), (3) or (4), the boron contained in the impurities is in a weight ratio of 0.
A cast steel containing less than or equal to 01% is provided.

[0014]

The present invention is a new high-temperature pressure vessel having excellent high-temperature characteristics by carefully selecting alloying elements using 12Cr-based steel as a basic component and striving to improve high-temperature strength while ensuring ductility, toughness and oxidation resistance. High chrome cast steel for use. The reasons for limiting the components in the high chromium cast steel for high temperature pressure vessels of the present invention will be described below.

Carbon: Carbon forms carbonitrides together with nitrogen and contributes to the improvement of creep rupture strength. But 0.0
If it is less than 5%, a sufficient effect cannot be obtained, and if it exceeds 0.13%, carbonitrides agglomerate and coarsen during use, deteriorating high-temperature long-term strength. Therefore, it is set to 0.05 to 0.13%.

Silicon: Generally, in this type of casting material, it is desirable that the silicon be slightly higher in order to secure castability, so according to the common sense, addition of up to 0.7% is recommended. I have decided to allow it. Addition of silicon improves the flow of the molten metal and has a calming effect on the molten metal, which is effective in preventing so-called casting defects. Instead, there is a problem that micro- and macro-segregation is likely to occur, making it difficult to obtain stable material properties.
The above-mentioned upper limit value of 0.7% allows this in a range in which the latter problem does not appear significantly.

Manganese: Manganese is an element useful as a deoxidizer. It also has the effect of suppressing the formation of δ ferrite. On the other hand, if this element is added in a large amount, the creep rupture strength deteriorates. Therefore, it is not preferable to add more than 1%. Further, especially when the content is suppressed to 0.1% or less, the creep rupture strength is further improved. For this reason, it is necessary to suppress the addition to 0.1% or less as needed.

Chromium: Chromium forms carbides and contributes to the improvement of creep rupture strength, and also dissolves in the matrix to improve the oxidation resistance, and strengthens the matrix itself to improve the strength at high temperature and long time. Contribute. 9.
If it is less than 5%, its effect is not sufficient, and if it exceeds 12%, δ-ferrite is produced and becomes cheaper, resulting in lower strength and toughness. Therefore, the component range is set to 9.5 to 12%.

Nickel: Nickel is an element effective in improving toughness. However, addition of a large amount significantly deteriorates the creep rupture strength. Therefore, addition of 0.6% or less is desirable. In particular, creep rupture strength is remarkably improved by adding it while suppressing it to 0.2% or less. For this reason, addition of 0.2% or less is necessary if necessary.

Vanadium: Hanadium becomes a carbonitride to improve the creep rupture strength. If it is less than 0.1%, a sufficient effect cannot be obtained. On the contrary, if the amount exceeds 0.3%, the creep rupture strength is rather lowered.
Therefore, the addition amount is set to 0.1 to 0.3%.

Niobium or Tantalum: Niobium or tantalum forms a carbonitride and contributes to the improvement of high temperature strength.
Also, contributing to prolonged improvements in creep rupture strength in the fine carbides (M ₂₃ C ₆₎ to precipitate at high temperatures. 0.01%
If it is less than 0.2%, it is not effective, and if it is added in an amount exceeding 0.2%, niobium or tantalum carbonitride produced during casting can be sufficiently dissolved in the matrix during heat treatment (solution treatment: 980 to 1150 ° C). Instead, it coarsens during use and reduces long-term creep rupture strength. Therefore, the total addition amount of these elements is set to 0.01 to 0.2%.

Nitrogen: Nitrogen forms carbonitrides with carbon and alloying elements and contributes to improvement of high temperature strength. If the amount is less than 0.01%, sufficient carbonitride cannot be formed, so that sufficient creep rupture strength cannot be obtained. On the other hand, if the amount added exceeds 0.1%, carbonitrides aggregate and coarsen over a long period of time, making it impossible to obtain sufficient creep rupture strength. Therefore, add 0.0
1 to 0.1%.

Molybdenum: Molybdenum forms a solid solution in the matrix together with tungsten to improve creep rupture strength. If molybdenum is added alone, it is possible to add about 1.5%, but when tungsten is added within the range of (1) of the present application, tungsten is more effective in improving high temperature strength, Further, when a large amount of molybdenum and tungsten are added, δ ferrite is formed and the creep rupture strength is deteriorated. Therefore, the amount of molybdenum is set to 0.5% or less in consideration of the balance with the amount of added tungsten.

Tungsten: Tungsten forms a solid solution in the matrix together with molybdenum as described above to improve creep rupture strength. Tungsten has a stronger solid solution strengthening function than molybdenum and is an effective element. However, if it is added in a large amount, δ ferrite and a large amount of Laves phase are generated, which conversely deteriorates the creep rupture strength. Therefore, considering the balance with the addition amount of molybdenum, 0.9
~ 3% addition.

Cobalt: Cobalt, like nickel, forms a solid solution in the matrix and suppresses the formation of δ-ferrite.
Further, it does not deteriorate high temperature strength unlike nickel. Therefore, when cobalt is added, it becomes possible to add more strengthening elements of chromium and tungsten than those without cobalt. As a result, high creep rupture strength can be obtained. However, if the amount added exceeds 2.5%, the precipitation of carbides is promoted, which deteriorates the creep rupture strength on the long-term side. Therefore, the addition is 2.5% or less.

Copper: Copper is contained as an unavoidable impurity and is effective as an element for suppressing δ ferrite.
Further, copper itself is finely precipitated in the matrix and is effective in improving high temperature strength. However, when copper is kept at a high temperature of over 1000 ° C., it causes grain boundary precipitation and causes embrittlement.
In addition, when the amount of copper deposited is large, the copper is coarsened while being kept at a high temperature, resulting in a decrease in creep strength for a long time. Therefore, addition of 1.2% or less of copper, which does not cause these adverse effects significantly, is allowed.

Boron: Boron is contained as an unavoidable impurity and has an effect of increasing the grain boundary strength. Therefore, it contributes to the improvement of creep rupture strength. However, if added in a large amount, the toughness decreases. For this reason, the addition amount is 0.
Keep it below 01%. In addition, unavoidable impurities including iron are included.

[0028]

EXAMPLES Examples of the present invention will be described below. Table 1
Is a chemical composition table comparing the test material and the comparative material which were actually tested. The present invention material is numbered 1-8, the comparative material is numbered 9-20, and the chemical composition is weighted horizontally. It is shown in%.

All materials were manufactured by melting in a 50 kg vacuum high frequency melting furnace and casting in a sand mold. Next, the test material was heated at 1050 ° C. for 10 hrs, cooled from 1050 ° C. to 300 ° C. at 73 ° C./hr assuming a large material such as a steam turbine casing, and then air-cooled for quenching treatment. . Then, tempering was performed by setting the tempering temperature of each material so that the 0.2% proof stress would be about 62 to 67 kgf / mm ² .

[0030]

[Table 1]

In Table 1, the comparative materials 19 and 20 show the components of the high chromium cast steel proposed in Japanese Patent Application No. 59-216322. In addition, those marked with * in the table indicate that the comparative material has a characteristically different component ratio from the material of the present invention. It is attached.

Table 2 shows the mechanical properties of the material of the present invention and the comparative material, which are given the same numbers as those in Table 1 above. In addition,
Comparative materials 19 and 20 are the same as those in Table 1 in Japanese Patent Application No. 59-2163.
No. 22 (Japanese Patent Laid-Open No. 61-96062).

[0033]

[Table 2]

The results of the room temperature tensile test show almost no difference between the material of the present invention and the comparative material, but the material numbers 11, 12, 16, 17, and 18 of the comparative material show low values in terms of impact characteristics. , (Column marked with * in the table) shows that the toughness is lower than that of the material of the present invention. Also material number 19,
Some of the present invention materials have a lower impact value than the 20 comparative materials, but in the sense of ensuring weldability, if the impact value is about the same as the present invention material, it is a particularly serious problem. I won't.

Table 3 shows 1 at 625 ° C. between the material of the present invention and the comparative material.
The creep rupture strength after 100,000 hours (extrapolated value from the time-breaking stress curve) is shown. As is clear from this result, the creep rupture strength of the material of the present invention is remarkably superior to that of all the comparative materials.

[0036]

[Table 3]

[0037]

As described above, the high chromium cast steel for a high temperature pressure vessel of the present invention has the same ductility and toughness as the conventional material as a result of carefully selecting the composition ratio of the alloy elements of the conventional material. In addition, the high temperature strength is dramatically increased. Therefore, it can be effectively utilized as a material for a turbine rotor casing or a valve chamber of an ultra-supercritical power generation plant having a steam temperature of more than 593 ° C. By using the cast steel of the present invention, the current ultra-supercritical power plant is further heated,
As a result, it contributes to the saving of fossil fuels and the amount of carbon dioxide generated can be suppressed to a low level, which is very useful in promoting the protection of the global environment.

Front page continuation (72) Inventor Yusaku Takano 5-717-1, Fukahori-cho, Nagasaki City Nagasaki Research Institute, Mitsubishi Heavy Industries, Ltd. (72) Inventor Ikujiro Kitagawa 46, Nakahara-Sennohama, Tobata-ku, Kitakyushu 59 Japan Cast and Forged Steel Co., Ltd. (72) Inventor Yasunori Tashiro, 59, Maharanohama, Nakahara, Tobata-ku, Kitakyushu City 59 Japan Cast and Forged Steel Co., Ltd. (72) Yukio Mochizuki, 46, Maharanohama, Nakahara, Tobata-ku, Kitakyushu Cast Forging Steel Co., Ltd.

Claims (5)

[Claims] 1. Carbon by weight: 0.05 to 0.13%,
Silicon: 0.7% or less, manganese: 1% or less, chromium: 9.5 to 12%, nickel: 0.6% or less, vanadium: 0.1 to 0.3%, total of niobium and tantalum:
0.01-0.2%, nitrogen: 0.01-0.1%, molybdenum: 0.5% or less, tungsten: 0.9-3%,
Cobalt: A high-strength, high-chromium cast steel for high-temperature pressure vessels, characterized in that it contains 2.5% or less and inevitable impurities and the balance is iron. 2. The high-strength, high-chromium cast steel for high-temperature pressure vessels according to claim 1, characterized in that manganese: 0.1% or less by weight ratio. 3. The high-strength, high-chromium cast steel for a high-temperature pressure vessel according to claim 1, wherein the weight ratio of nickel is 0.2% or less. 4. Copper: 1.2% by weight in the impurities.
A high-strength, high-chromium cast steel for a high-temperature pressure vessel according to claim 1, wherein the patent includes the following. 5. Boron: 0.0 by weight in the impurities.
The high-strength, high-chromium cast steel for a high-temperature pressure vessel according to claim 1, 2, 3 or 4, which contains 1% or less.