KR100410699B1 - High and Low Pressure Integrated Rotor Alloy Steel for Steam Turbine - Google Patents

Abstract

The present invention relates to an integral rota alloy steel for steam turbines having excellent impact toughness and high temperature creep strength, and in particular, 0.15 to 0.45 wt% carbon, 0.005 wt% or less phosphorus, 0.005 wt% or less sulfur, 0.05 wt% or less manganese, silicon 0.05 wt% or less, nickel 1.0-2.0 wt%, chromium 1.0-3.0 wt%, molybdenum 1.0-2.0 wt%, vanadium 0.1-0.5 wt%, navidium 0.01-0.05 wt%, tungsten 0.1-0.4 wt%, boron 0.001 To 0.005% by weight, the sum of phosphorus, sulfur, manganese and silicon is 0.2% by weight or less, and the remainder is iron and alloy iron or impurities inevitably incorporated during the manufacturing process, and 0.001% by weight or less of tin and arsenic 0.001% by weight or less. , With a chemical composition of less than 0.02% by weight of aluminum, as much as possible to suppress brittle elements such as phosphorus, sulfur, manganese, and silicon, compared to steels that have been conventionally used as integral rota materials for steam turbines It is characterized in that the impact toughness and high temperature creep strength are improved by improving the impact toughness and high temperature creep strength, which are the problems of the integral rota material by optimally adding Naobium, tungsten and boron. By increasing the temperature to 566 ° C, not only the fuel cost and pollution emission reduction due to the efficiency increase, but also the power generation cost can be reduced, as well as an additional import replacement effect.

Description

High and Low Pressure Integrated Rotor Alloy Steel for Steam Turbine}

The present invention relates to an alloy steel that satisfies both properties simultaneously to improve the impact toughness and high temperature creep properties of rota alloy steel used in the high pressure and low pressure portions of a steam turbine, so that the high pressure and low pressure portions can be used in one piece. To the steel containing (Cr) 1.0 to 3.0% by weight, elements such as nickel (Ni), molybdenum (Mo), vanadium (V), navy (Nb), tungsten (W) and boron (B) are added, The present invention relates to a rota alloy steel in which phosphorus (P), sulfur (S), manganese (Mn) and silicon (Si) are regulated to a total weight of 0.2% by weight or less, further improving impact toughness and high temperature creep properties.

As a material for the rotor for general steam turbines, 1CrMoV steel having excellent high temperature creep strength has been mainly used in the high pressure section, and 3.5 NiCrMoV steel having excellent toughness has been used in the low pressure section. By manufacturing each rota used in the low pressure part and the one-piece rota, the coupling is eliminated and the length of the rota is reduced, and there is no need for the machinery or hardware required to manufacture the coupling.

Therefore, it has advantages in all aspects such as reduction of required site, manufacturing cost, and reduction of installation / operation cost, but it is very difficult to develop a steel grade that satisfies both characteristics required when making the high pressure part and the low pressure part in one piece. There are disadvantages.

In particular, the high pressure portion requires high temperature creep strength, and the low pressure portion requires the opposite property of requiring toughness, and thus it is not easy to develop a material having a creep strength equivalent to that of conventional 1CrMoV steel and having toughness similar to that of 3.5 NiCrMoV steel.

In addition, the conventional rotor rotor material for steam turbine is not only low impact toughness, but also due to the problem of temper brittleness due to long time use, the increase in ductility-brittle transition temperature (FATT) and low temperature creep properties Therefore, in order to operate the high pressure and low pressure integrated rotor for steam turbine stably, development of an alloy steel material having improved high temperature creep strength and impact toughness is required.

Thus, in order to satisfy the two opposite requirements of high temperature creep strength for toughness and toughness for use in the low pressure part, 1CrMoV steel (ASTM A 470 Class 8), which is a material of a high pressure rotor, and a low pressure rotor, Combination of 3.5 NiCrMoV steel (ASTM A 470Class 7) with 2.25CrMoV steel or 1.8NiCrMoV steel to add Navium (Nb) and tantalum (Ta) to improve creep strength, toughness, calcium (Ca) and cesium By adding and combining (Ce) and rare earth elements (REM: Rare Earth Metal), alloy steels which spheroidized non-metallic inclusions such as oxides and sulfides to improve cleanliness and mechanical properties have been developed.

However, the alloy steel has a creep strength lower than that of 1CrMoV steel, and when used for a long time, the degradation of toughness due to temper brittleness due to impurity elements could not be prevented.

That is, only elements contained in the conventional rota steel for steam turbines have a limit in improving high temperature creep strength and impact toughness at the same time, and impurity elements such as phosphorus (P) and sulfur (S) are used as grain boundaries for a long time. Segregation at the brittle and brittle constituent elements such as manganese (Mn) and silicon (Si) further enhance the grain boundary segregation of phosphorus (P) and sulfur (S) elements, leading to soft-brittle transition during the use of Rota steel. There is a problem in that a significant economic loss occurs because the temperature is increased early deterioration or the possibility of cracking, the design life can not be used and replacement is inevitable.

The present invention has been made to improve the problems of the conventional rotor material for steam turbines as described above, to improve the high temperature creep in steel containing 1.0 to 3.0% by weight of chromium (Cr), Nab (Nb), tungsten (W) and boron (B) elements are added, and phosphorus (P), sulfur (S), manganese (Mn) and silicon (Si) are regulated to improve high temperature creep strength and impact toughness simultaneously, even when used for a long time. An object of the present invention is to provide an integral rota alloy steel for a steam turbine in which the high temperature creep strength and impact toughness do not decrease.

1 is a graph showing the relationship between yield strength and ductility-brittle transition temperature of alloy steel and comparative steel according to the present invention,

2 is a graph showing the relationship between creep rupture strength and Larson-Miller parameter of alloy steels and comparative steels according to the present invention.

In order to achieve the above object, the present invention is 0.15 to 0.45% by weight of carbon (C), 0.005% by weight or less of phosphorus (P), 0.005% by weight or less of sulfur (S), 0.05% by weight or less of manganese (Mn), 0.05 wt% or less of silicon (Si), 1.0 to 2.0 wt% of nickel (Ni), 1.0 to 3.0 wt% of chromium (Cr), 1.0 to 2.0 wt% of molybdenum (Mo), 0.1 to 0.5 wt% of vanadium (V), age Empty (Nb) 0.01 to 0.05% by weight, tungsten (W) 0.1 to 0.4% by weight, boron (B) 0.001 to 0.005% by weight, the remainder is iron and ferroalloy or impurities inevitably mixed during the manufacturing process Tin (Sn) 0.001% by weight or less, arsenic (As) 0.001% by weight or less, aluminum (Al) 0.02% by weight of the chemical composition is characterized by using a high pressure and low pressure integrated rota material for steam turbines.

Hereinafter, the reasons for limiting the respective components of the integral rota steel for steam turbines composed of the chemical composition as described above are as follows.

1. Carbon (C)

Carbon supersaturated in steel precipitates as carbide during quenching and tempering, causing precipitation strengthening and improving high temperature creep properties. In order to improve the ductile-brittle transition temperature, it is necessary to suppress the tempering process or temporary carbide precipitation on the short time side, and to deposit fine carbide on the long time side, the carbon content should be low. Toughness deteriorates, and when the content becomes 0.45% by weight or more, coarse carbides are formed, and thus the high temperature creep property is reduced, so that the carbon content is 0.15 to 0.45% by weight.

2. Molybdenum (Mo)

Molybdenum contributes to strength improvement by forming carbides that are effective and stable in improving high temperature strength through solid solution strengthening and precipitation strengthening. This maintains the solid solution effect and the microprecipitation effect of molybdenum for a long time to improve the creep strength at high temperature and long time. When the addition amount is large, the precipitate is coarsened and the toughness may occur and the molybdenum content is 1.0 to 2.0% by weight because it is uneconomical.

3. Vanadium (V)

Vanadium is effective in improving high temperature strength through solid solution strengthening and precipitation strengthening, and precipitates many carbides during tempering. If the amount of vanadium is large, the vanadium consumes all of the carbon in the matrix, making it difficult to continuously precipitate M ₂₃ C ₆ carbides or other forms of carbide during use, and if the amount of vanadium is small, the deposition place is reduced and M ₂₃ C ₆ Since it is difficult to uniformly distribute carbides and the coarsening becomes easy as a result, the vanadium content is 0.1 to 0.5% by weight.

4. Nb

Since navirium has little solid solubility at the base, most of the added amount is present as carbides. Fine nacicarbide reduces the ductile-brittle transition temperature by miniaturizing grain size. In case of adding a small amount of navium, low temperature toughness and creep strength are improved by synergistic effect of strengthening VN dispersion precipitation, but adding a large amount of coarse Nb (C) process carbide precipitates to reduce toughness. Therefore, the navy content is set at 0.01 to 0.05% by weight.

5. Chrome (Cr)

Chromium contributes to high temperature strength through solid solution strengthening and precipitation strengthening, and improves strength and toughness as a major element of M ₂₃ C ₆ carbide formation. The addition of chromium increases the corrosion resistance and oxidation resistance and contributes to the improvement of high temperature strength. However, in the case of low alloy steel, the chromium content is 3.0% by weight in consideration of economical efficiency. When the chromium content is less than 1.0 wt%, there is less carbide required to maintain high temperature strength, so the chromium content is 1.0 to 3.0 wt% to maintain strength and toughness.

6. Nickel (Ni)

The addition of nickel improves toughness, but the addition of nickel lowers the austenite starting temperature, which accelerates the recovery of the matrix structure in the high temperature creep test, thereby reducing the creep rupture strength. However, if the maximum strength exceeds 2.0 wt% in terms of creep strength, a sudden drop in creep strength may occur, so the nickel content is 1.0 to 2.0 wt%, which is a range in which the creep strength is not reduced and the toughness can be improved. .

7. Tungsten (W)

Tungsten is substituted and dissolved in iron, and some of it is added to the matrix to delay the recovery of the matrix, and some form precipitates such as W ₂ C, Fe ₂ W, and M ₂₃ C ₆ and M ₆ C carbides. Some are employed to delay the coarsening of carbides. Tungsten makes a great contribution to improving creep strength, but as the amount of addition increases, the toughness rapidly increases due to the generation of a Laves phase, a generic term for a group of intermetallic compounds with a dense filling structure of type AB ₂ on the short-term side. Since it lowers, tungsten content shall be 0.1-0.4 weight%.

8. Boron (B)

Boron is employed as iron invasive or substituted, and exists as boron carbide or boride to strengthen grain boundaries. Boron is an effective element to improve the hardenability and high temperature creep strength, but if it is more than the proper amount, it is adversely affected, and it is difficult to remove by easily combining with nitrogen and oxygen. Since boron shows the maximum creep strength at 0.0012% by weight and shows the highest creep ductility at 0.0048% by weight, the boron content is 0.001 to 0.005% by weight.

9. Phosphorus (P) and Sulfur (S)

Phosphorus and sulfur reduce the strength and impact energy in materials used at high temperatures, and the maximum content is limited because they contribute greatly to the prevention of temper embrittlement as grain boundary segregation. In order to prevent temper embrittlement, phosphorus is limited to about 0.005% by weight at maximum and sulfur is at most about 0.005% by weight.

10. Manganese (Mn) and Silicon (Si)

Manganese is effective in improving the hot workability of steel and stabilizing the structure, but when it is contained in a large amount, it combines with sulfur to form MnS inclusions to reduce toughness and promote temper brittleness. In addition, silicon acts as a deoxidizer when steelmaking and improves oxidation resistance, but if it is added a lot, the toughness lowers and the creep strength should be kept as low as possible. Therefore, in order to prevent temper embrittlement, manganese and silicon are respectively limited to 0.05 wt% at maximum.

11.Aluminum (Al), Tin (Sn), Arsenic (As)

Aluminum is added for deoxidation in steelmaking and combines with nitrogen and oxygen to inhibit grain growth. In addition, there is little effect on creep rupture strength and ductility, but if an appropriate amount is present, all of the dissolved nitrogen is consumed as AIN, and the creep strength is drastically reduced. Therefore, the maximum amount of Al remaining as the deoxidizer should be less than the value required for the stoichiometric bonding of N as AIN, so it is limited to a maximum of 0.02% by weight. As a segregation to lower the impact resistance as well as causing temper brittleness, in the present invention, in order to prevent this, the tin and arsenic must be limited to 0.001% by weight or less.

(Example)

Using the Rota steel of the present invention and the Rota steel which has been used conventionally as a comparative steel, various characteristics were tested and compared with each other.

In the following Table 1, Comparative steel 1 is a steel of 1CrMoV, which was conventionally used as a high pressure rota steel for a steam turbine, Comparative steel 2 is a 3.5NiCrMoV steel used as a low pressure rota steel, Comparative steel 3 is an integral rota steel Steels of Japanese Patent Application Laid-Open No. 09-194946 and Comparative Steel 4 are steels of Japanese Patent Application Laid-open No. Hei 10-183294.

The alloy as shown in Table 1 was dissolved in an electric furnace, and forged ingot into a square bar shape having a width of 100 mm, a thickness of 50 mm, and a length of 300 mm in the range of 950 to 1200 ° C., followed by 20 hours at 850 ° C. Annealing treatment was performed. The heat treatment was heated at 900 to 1050 캜, immersed in water and quenched, and then, the tempering heat treatment was heated to 500 to 700 캜 and then slowly cooled to impart ductility and toughness to the material. Tensile test pieces, impact test pieces, and creep test pieces were taken to investigate the mechanical properties after heat treatment.

Table 2 below shows the results of testing the mechanical properties of the comparative steel and the inventive steel.

As can be seen in Table 2, in the present invention, the tensile strength and the ductile-brittle transition temperature are superior to those of the conventional comparative steel.

1 is a graph showing the relationship between the yield strength and the ductility-brittle transition temperature of the conventional comparative steel and the present invention steel, as shown, when comparing the ductility-brittle transition temperature to yield strength, the conventional comparative steel is equivalent yield While the ductile-brittle transition temperature is high in strength, the steel of the present invention has high impact strength with low yield strength and low ductility-brittle transition temperature.

Figure 2 is a graph showing the relationship between the creep strength of the conventional comparative steel and the inventive steel according to time and temperature by using the Larson-Miller parameter, which shows the creep rupture strength on the vertical axis and the temperature and time on the horizontal axis as one parameter. As a result, the dashed-dotted line indicates the creep rupture strength of 1CrMoV steel (ASTM A 470 Class 8), which is a conventional high-pressure part material, and the dotted line is the result of the creep rupture strength of the comparative steel 3, and the solid line is the creep rupture of the inventive steels 1-4. It shows the strength,

As can be seen in FIG. 2, the creep rupture strength of the inventive steel is 150 MPa and 95 MPa for the comparative steel at a value of 20.98 corresponding to a Lason-miller parameter of 566 ° C. and 100,000 hours. On the other hand, considering that the required design strength at 566 ° C. and 100,000 hours is at least 100 MPa, the present invention steel has better creep rupture strength than conventional comparative steels, and can be used as a material for integral rotors for steam turbines even at 566 ° C. It shows the strength that can be.

As described above, the inventive steel reduces the brittle and brittle constituent elements phosphorus (P), sulfur (S), manganese (Mn) and silicon (Si) in the comparative steels that have been used in the past, Optimum addition of (Nb), tungsten (W) and boron (B) prevents the drop in impact toughness and high temperature creep properties due to the increase in ductile-brittle transition temperature, which was the biggest problem of conventional comparative steels. It was possible to produce 1.0 to 3.0% by weight of chromium steel having excellent impact toughness.

According to the integrated rota alloy steel for steam turbine of the present invention having the above object and configuration, as compared with steels that have been used in the past, brittle coarse elements such as phosphorus, sulfur, manganese, and silicon are suppressed as much as possible. By adding boron optimally, the impact toughness and high temperature creep strength are prevented from increasing the ductile-brittle transition temperature, which is a problem of the integral rota material for steam turbines, and thus the high temperature creep strength is excellent and the impact toughness is also excellent.

In addition, due to the improvement of the high temperature creep properties, the operating temperature of the integral Rota material for steam turbine is raised from 538 ℃ to 566 ℃, thereby reducing the power generation cost by reducing fuel cost and suppressing pollutant emissions due to increased efficiency, and incidentally imported Substitution effects can also be obtained.

Claims (2)

0.15 to 0.45 wt% of carbon (C), 0.005 wt% or less of phosphorus (P), 0.005 wt% or less of sulfur (S), 0.05 wt% or less of manganese (Mn), 0.05 wt% or less of silicon (Si), nickel (Ni) 1.0 to 2.0% by weight, chromium (Cr) 1.0 to 3.0% by weight, molybdenum (Mo) 1.0 to 2.0% by weight, vanadium (V) 0.1 to 0.5% by weight, Nabium (Nb) 0.01 to 0.05% by weight, tungsten (W) ) 0.1 to 0.4% by weight, boron (B) 0.001 to 0.005% by weight, the balance is iron and ferroalloy or impurities inevitably incorporated during the manufacturing process Tin (Sn) 0.001% by weight or less, arsenic (As) 0.001 A high-pressure and low-pressure integrated rota alloy steel for steam turbines, characterized in that the weight% or less, aluminum (Al) 0.02% or less. delete