JPH0459944A - Low alloy steel for steam turbine rotor - Google Patents

Abstract

PURPOSE:To manufacture a low alloy steel for a turbine rotor integrating both high and low pressure usages having high temp. strength in a high pressure part and an intermediate pressure part and having excellent toughness, tensile strength and proof stress in a low pressure part by preparing a steel having a specified compsn. in which each content of Cr and Mo is prescribed. CONSTITUTION:A low alloy steel having an alloy compsn. contg., by weight, 0.15 to 0.35% carbon, <=0.15% silicon, <=12% manganese, 0.5 to 2.0% nickel, 3.5 to 4.5% chromium, 0.5 to 1.5% molybdenum, 0.5 to 1.0% tungsten and 0.1 to 0.35% vanadium, furthermore contg., at need, either one kind of <=0.1% niobium and <=0.1% tantalum and the balance iron with inevitable impurities is prepd. In this way, a rotor material excellent in strength at a room temp. and toughness, high in reliability compared to that of a conventional one and furthermore, suitable for a larger high-low pressure integrated steam turbine rotor can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a low alloy steel for high and low pressure integrated steam turbine rotors, and more particularly, the part exposed to low temperature steam has excellent toughness and strength. The parts exposed to high temperature steam also relate to low alloy steels used as materials for forming high and low pressure integrated steam turbine rotors, each having excellent high temperature creep rupture strength.

Conventional technology Large forged products of low-alloy steel are used for the rotor material of steam turbines, but as the capacity of steam turbine plants increases in recent years, due to restrictions on the properties of the rotor material, steam turbines have to be manufactured under high pressure. Manufactured in two compartments: side and low pressure side. As shown in Table 1, for example, as shown in Table 1, the rotor material of the high-pressure side casing has a chemical composition with excellent high-temperature strength (e.g., creep rupture strength). The rotor material of the low-pressure side casing is made of a material having a chemical composition with excellent room temperature strength (for example, yield strength) and toughness (for example, Charpy impact value).

Furthermore, in large-capacity steam turbine plants, the casings are divided into three casings: high pressure, intermediate pressure, and low pressure.
Materials with chemical compositions that have excellent high-temperature strength at temperatures around room temperature (up to 0° C.), excellent high-temperature strength at temperatures around room temperature, and excellent toughness at temperatures around room temperature are used.

As mentioned above, when producing a steam turbine divided into two or three casings using two or three types of rotor materials depending on the characteristics of each casing, the high cost of the steam turbine can be reduced. Invitation,
It is extremely disadvantageous economically.

In particular, for rotor materials for small and medium-sized power generation steam turbines in the 5050-2O0 class, the same material is used from the high-pressure side to the low-pressure side from the viewpoint of downsizing the plant, simplifying the mechanism, and reducing the site area. It is considered to be formed using Chemical compositions of rotor materials for such purposes include those shown in Table 2 below. However, these rotor materials have also been affected by the recent rise in the temperature of steam turbines (the steam temperature used in conventional power generation steam turbines is at most 500°C) and the increase in the size of steam (the capacity of conventional power generation steam turbines is 75 MW at most). Its properties were insufficient to meet the trend of increasing the size of steam turbines in combined power generation plants using gas turbines and steam turbines.

Therefore, intensive research has been conducted to develop rotor materials for high and low pressure integrated steam turbines having a chemical composition that has strength at both high and normal temperatures and excellent toughness even at room temperatures. As a result, 2
・'/a CrMoV steel was proposed (Special Publication Publication 1986-19)
370), and furthermore, this 2·′/4CrMoV steel is
A rotor material for high and low pressure type steam turbines made of low alloy steel added with b has also been proposed (Japanese Patent Laid-Open No. 60-24577).
No. 2). In addition, as already publicly known, CrMoV
There are also steels with Nb added (Special Publication No. 58-1150)
No. 4). These 2-'/4CrMoV steel, 2.1/4C
The chemical compositions of high-low pressure integrated low-alloy steels, which are rMoVNb steels and CrMoV steels with Nb added, are also shown in Table 2 below.

Problems to be Solved by the Invention In such a rotor for a high and low pressure integrated turbine for a steam turbine for power generation, it is necessary to have high temperature strength in the high pressure section and intermediate pressure section, and to have toughness and toughness in the low pressure section. It is necessary to have excellent tensile strength and yield strength.

Conventionally proposed high and low pressure integrated rotor materials having chemical compositions as shown in Table 2 have the following disadvantages.

In other words, in a rotor having a chemical composition shown in Table 2, if the rotor diameter is approximately 1600 mm or more, the high-temperature strength of the high-pressure part will not be sufficient, and sufficient hardenability will not be obtained, and the low-pressure part will not have enough strength. It has drawbacks such as insufficient toughness in the center.

Means for Solving the Problem The inventors achieved the following by using a specific alloy composition as described below.
Compared to the material properties of the rotor with the chemical composition shown in Table 2, it can exhibit excellent strength characteristics in the high-pressure section and low-pressure section, as well as excellent toughness in the low-pressure section, ensuring that the desired properties are always exhibited. The inventors have discovered that it is possible to provide a high-strength, high-toughness, high-low-pressure integrated rotor material for a steam turbine, and have thus arrived at the present invention.

Therefore, the low alloy steel for high and low pressure integrated steam turbine rotor according to the present invention has, on a weight basis, carbon 0.15035%, silicon 0.15% or less, manganese 12% or less, nickel 0.
5-2.0%, chromium 3.5-4.5%, molybdenum 0
．． 5-1.5%, tungsten 0.5-1.0%, vanadium 0.1-0.35%, and if necessary, niobium 0.1% or less or tantalum 01% or less. It is characterized by having an alloy composition containing any one of these elements, with the balance consisting of iron and inevitable impurity elements, and the content of Ni + Cr + Mo + V is more than 565% and 7.3% or less.

Compared to the low-alloy steels according to the three inventions mentioned above, the low-alloy steel of the present invention contains more of the elements Ni and Cr. This is because the power generation capacity of high and low pressure integrated steam turbine plants has increased in recent years (previously it was at most 75 MW, but in recent years it has increased to 200 MW), and as a result the rotor material has also become larger, improving hardenability. The need arose;
This is because more alloying elements are needed to improve hardenability.

Effects The reason for limiting the composition and content (weight basis) of the low alloy steel of the present invention as described above will be described below.

C is an essential element for increasing hardenability and ensuring yield strength and toughness. 0.15% or more is necessary to develop the proof strength and toughness required for the rotor material according to the present invention, but if too much is added, the toughness will be adversely affected and workability will be deteriorated, so the content 0
．． 15-0.35%.

Arrow 1st Sl is an element effective as a deoxidizing agent for molten steel.

However, when a large amount of Si is added, Si, which is a product of deoxidation, remains in the steel, impairing the cleanliness of the steel, reducing toughness, and decreasing creep rupture elongation (ductility).
Furthermore, since it promotes tempering brittleness during high-temperature use, its content is set to 0.15% or less.

In addition, in recent years, vacuum carbon deoxidation method and electroslag remelting method have been applied to rotor materials, and it is no longer necessary to perform Si deoxidation, and the amount of Si can be reduced.

Mn is effective as a deoxidizing agent and desulfurizing agent for molten steel, and is also an effective element for increasing hardenability and strength. However, adding too much impairs toughness and ductility, so its content was set at 12% at most.

Ni1 is an element that is effective in increasing the hardenability of steel and increasing its strength and toughness at room temperature, and is particularly effective in improving toughness. Further, these effects are significantly increased when the contents of both Ni and Cr are large due to their synergistic effect. However, if too much Ni is added, high temperature strength (
Creep strength, creep rupture strength) and promotes temper brittleness, so its content should be reduced to 0.5-2.0.
%.

12nd Cr is the most important element as an additive element in ordinary low alloy steel for rotors. Adding Cr to steel improves corrosion resistance,
Improves oxidation resistance, increases hardenability and improves tensile properties at room temperature. Further, these effects are significantly increased due to a synergistic effect when the contents of both Ni and Cr are large. Furthermore, Cr is an element that is effective in improving lift strength such as creep strength and creep rupture strength. However, if the content exceeds 4.5%, it is difficult to make a significant improvement, so there is no need to add a large amount.

Furthermore, since the hardenability (bainite hardenability) is improved as described above, the content was set at 3.5-4.5% in consideration of the mass effect of the rotor material. Incidentally, in the past, it has been said that the optimum Cr content for high-temperature rotor materials is around 1%, but from the perspective of improving toughness, in the case of larger rotor materials, the above-mentioned Cr content should be slightly higher, considering hardenability. is optimal.

Similar to Cr, Mo is an important element added to ordinary low-alloy steel for rotors. Adding Mo to steel increases hardenability, increases temper softening resistance during tempering, and is effective in increasing room temperature strength (tensile strength, yield strength). In addition, MO is a solid solution strengthening element that produces carbides and acts as a precipitation effecting element, and is a very effective element for improving high temperature strength such as creep strength and creep rupture strength. Furthermore, if MO is added at about 0.5% or more,
It is an extremely effective element for preventing steel from becoming brittle during tempering. However, if too much is added, the effect will be saturated and the toughness will be adversely affected. Moreover, Mo is an expensive element, and adding too much will increase the cost. Therefore, the mass effect (hardenability) when the rotor material becomes larger
Considering that, Moff1 is set to 0.5-1. ．． It was set at 5%.

As a solid solution strengthening element, W is a very effective element for improving high temperature strength such as creep strength and creep rupture strength. However, if too much is added, phenomena such as solidification segregation that are undesirable for large castings will occur, so the amount of W should be reduced to 0.
．． It was set at 5-1.0%.

Like Mo, Yuro 2Ei No. 4D is an effective element for improving strength (tensile strength, proof stress) at room temperature, and also as a solid solution strengthening element and a precipitation hardening element that produces carbides.
It is an important element that increases high-temperature strength such as creep strength and creep rupture strength. Furthermore, if ■ is within a certain addition range (003-035%),
It is also effective in improving toughness by making crystal grains finer. However, adding too much will harm the toughness and it is an expensive element, resulting in high costs, so the content should be reduced to 0.1-0.
．． It was set at 35%.

Main Examples Main P, 5SCu, etc. are mixed in as impurity elements from the raw materials for steelmaking and are unavoidable, but it is desirable that their content be as low as possible. However, if the raw materials are carefully selected, the cost will be high, so P is less than 0.015% and S is 0.0%.
10% or less, Cu is desirably 0.50% or less, and other impurity elements include A1 and Sn.

Examples include Sb, Pb, As, etc.

In the low-alloy steel for a steam turbine rotor according to the present invention, in addition to the above-mentioned essential constituent elements, any one of niobium and tantalum can be added as an optional component if necessary.

The reason for limiting the content of these arbitrary elements is as follows.

Like ■, niobium (Nb) is an element that is effective in increasing room-temperature strengths such as tensile strength and proof stress, as well as high-temperature strengths such as creep strength and creep rupture strength. Although it is a very effective element for improving
．． If it is less than 0.01%, the effect is not sufficient. In the rotor material of the present invention, the addition of Nb is not expected to increase the strength much;
It is expected that the toughness will be improved by grain refinement, but if too much is added, a large amount of Nb carbide will be formed, which will actually impair the toughness and will not be effective. Therefore, the Nb content is 01%
The following was made.

Tantalum (Ta) also has the same effect as Nb.

The low alloy steel according to the present invention is quenched at a temperature of 900-1050°C and tempered at a temperature of 550-750°C, and has higher high-temperature strength than conventional rotor materials in the part exposed to high-temperature steam (high pressure side). It also shows notch reinforcement in creep rupture tests (smooth creep rupture time is shorter than notch creep rupture time), and the parts exposed to low-temperature steam (low pressure side) have excellent strength (yield strength) even at room temperature. The present invention provides rotor materials for high and low pressure type turbines that have high and toughness.

Examples of the present invention are shown below.

EXAMPLE The low alloy steel of the present invention having the chemical composition shown in Table 3 below was melted in a laboratory scale vacuum melting furnace to produce a 50 kg steel ingot.

From these steel ingots, a small forged material was produced by heating the actual rotor and forging process (upsetting 1/2.8U, forging elongation 3.7S forging). Thereafter, this forged material was subjected to preliminary heat treatment (for example, air cooling at 1010° C. and air cooling at 720° C.) for the purpose of adjusting the grain size. The maximum diameter of the high-pressure part of the high-low pressure integrated rotor material is φ1600m.
Maximum diameter of the part corresponding to the low pressure part during forced air cooling of m φ20
It was subjected to heat treatment simulating the quenching cooling rate of the center and outer circumference during water quenching of 0.00 mm.

In other words, as heat treatment for the part corresponding to the high pressure part, 950
After heating at ℃ to completely austenitize, ・Quenching cooling rate at the center of high pressure part (average of 950℃-300℃): Approx. 2
5℃/Hr, (specimen material A) Quenching cooling rate of outer periphery of high pressure part (average of 950℃-300℃): Approximately 75℃/H
After quenching at two cooling rates: r and (designated as sample material B), these sample materials A and B were tempered at 650°C. In addition, the above sample materials Δ and B were tempered,
The strength required for the design of the high-pressure part of the high-low pressure integrated steam turbine rotor, that is, the 0.2% yield strength, was adjusted to be ~70 Kg/mm''.

As a heat treatment for the part corresponding to the low pressure part, after heating it at 900℃ to completely austenitize, C)・Quenching cooling rate of the outer periphery of the low pressure part (average of 900°C-300°C);
After quenching at two cooling rates of approximately 1600°C/Hr (test materials), these test materials C and D were heated to 650 °C/Hr.
Tempered at ℃. In addition, the above test materials C and D were tempered to have a strength necessary for the design of the low pressure part of a high and low pressure integrated steam turbine rotor, that is, a 0.2% yield strength of ~7.
It was adjusted to 7Kg/mm2.

In addition to the low alloy steel of the present invention, the current 2.1/4Cr
Molten MoV steel in the same manner as above (comparative material E)
, the properties of these low alloy steels were compared.

Table 4 shows the results of the tensile test and impact test of test materials A to D related to the low alloy steel of the present invention, and Table 5 shows the results of the creep rupture test.

! l1 Creep rupture test results for sample materials In addition, the creep rupture strength was organized using Larson-Miller parameters, and the creep data sheet No. 9A (conventional high-pressure rotor material CrM) of the Institute of Materials Technology
Figure 1 shows a comparison with the data for oV steel. That is,
Figure 1 shows the Larson-Miller parameter [T (20+
log t)
Kg/mm') in the graph, ○ in the graph indicates sample material A corresponding to the center of the high-pressure part, and . indicates sample B corresponding to the outer periphery of the high-pressure part.

As is clear from Figure 1, the 02% yield strength of specimens A and B, which correspond to the high pressure section, is 70 kg/mm" or more, and the 0.2% yield strength of specimens C and D, which correspond to the low pressure section, is 11 kg/mm".
It has a strength level of 2 or more, and has sufficient strength as a high-low pressure integrated steam turbine rotor. Furthermore, the elongation and the reduction of area fully satisfy the requirements for a general low-pressure rotor, such as an elongation of 16% or more and a reduction of 45% or more.

Regarding impact resistance, the 50% FATT on the low pressure side of the conventional high/low pressure integrated rotor is +80°C or less, but in the future, a lower FATT will be required to reduce starting and stopping costs. Comparison material E is the current 2.'la CrMoV steel high and low pressure integrated steam turbine rotor, but the FATT of comparison material E in the center of the low pressure section is as high as 68°C, and the high and low pressure type steam turbine rotor is larger. In this case, there is a slight problem with reliability regarding toughness, and improvement is required. On the other hand, in the low alloy steel according to the present invention, the test material A was heated to 140°C1
Test material B was 18°C, test material C was +22°C, and test material was -115°C, indicating a decrease in FATT and a significant improvement in toughness.

As is clear from this, the cuff resistance is 0-80Kg/mm.
It has relatively excellent strength at room temperature and improved toughness (50% FATT in the center of general high-pressure rotor material: 8012
0°C), and creep rupture strength was found to have properties preferable as a conventional high-pressure rotor material.

In the creep rupture test, the smooth-notched combination test piece shown in FIG. 2 was used, and under all test conditions, the specimen broke at the smooth portion, indicating good notch reinforcement.

Effects of the Invention As described above, the low alloy steel of the present invention has excellent room temperature strength and toughness, is more reliable than conventional steels, and is a rotor material suitable for larger high and low pressure integrated steam turbine rotors. Obtainable.

Depending on the application, the low alloy steel of the present invention may not necessarily be used as a high-low pressure integrated steam turbine rotor, but may also be used as a rotor material for only the low-pressure section or only the high-pressure section.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between stress and the Larson-Miller parameter of the creep rupture strength of the rotor material of the present invention, Figure 2
The figure is a cross-sectional view of a creep rupture test piece (smooth-notched combination type) used for Larson-Miller parameter measurements.

Claims (1)

[Claims] By weight: 0.15-0.35% carbon, 0.1% silicon
5% or less, manganese 1.2% or less, nickel 0.5-2
．． 0%, chromium 3.5-4.5%, molybdenum 0.5-
1.5%, tungsten 0.5-1.0%, vanadium 0.1-0.35%, and if necessary, niobium 0.1% or less or tantalum 0.1% or less. Contains one of these, the remainder consists of iron and unavoidable impurity elements, and the content of Ni + Cr + Mo + V is 5.5
Low alloy steel for steam turbine rotor, characterized in that it has an alloy composition of more than 7.3% and less than or equal to 7.3%.