JPH0941076A - High strength and high toughness low alloy steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To develop a high-low pressure integrated rotor material for a steam turbine solo chamber excellent in creep fracture strength at a high temp. and usable at a high temp. by adding specified small amounts Co and Ti to a high strength and high toughness low alloy steel having a specified compsn. SOLUTION: A low alloy steel having a compsn. contg., by weight, 0.1 to 0.3% C, <0.6% Si, <0.1% Mn, 0.1 to 1.5% Ni, 0.5 to 3% Cr, 0.05 to 0.5% Mo, 0.1 to 0.35% V, 0.01 to 0.15% Nb, 0.5 to 2% W and 0.001 to 0.01% B, in which Mo equivalent expressed by Mo+1/2W(%) is regulated to 0.7 to 1.4, and the balance Fe or having a compsn. furthermore added with <2% Co and <0.05% Ti is melted in a vacuum melting furnace or the like to develop the high strength and high toughness low alloy steel suitable as a rotor material on the high pressure side and low pressure side of a steam turbine, a rotor material of a gas turbine and a disk material, having excellent high temp. strength in the high pressure part and excellent room temp. strength and toughness in the low pressure part and sufficiently usable in the high temp. region of 565 deg.C.

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-toughness low-alloy steel, and more particularly to a turbine casing of a steam turbine, a high-low pressure integrated rotor material, a high-pressure side of a steam turbine, a low-pressure side rotor material, and a rotor material such as a gas turbine. The present invention relates to the same low alloy steel that can be advantageously applied to disc materials.

[0002]

2. Description of the Related Art A large forged product of low alloy steel is used as a rotor material of a steam turbine. In this case, although depending on the capacity of the steam turbine, the rotor material is manufactured in two high-pressure and low-pressure compartments, or in a large-capacity steam turbine, divided into high-pressure, medium-pressure and low-pressure compartments. The rotor material for the high-pressure vehicle interior is made of a material having excellent high-temperature strength (for example, creep rupture strength), and the rotor material for the low-pressure vehicle interior is at room temperature strength (for example, yield strength) and toughness (for example, V Charpy impact value). Excellent materials are used.

However, when the steam turbine is divided into two or three compartments as described above, the cost of the steam turbine increases, which is economically extremely disadvantageous. Especially 50
From the standpoints of downsizing of plant, simplification of mechanism, reduction of site area, etc., medium to small steam turbines of 200 MW class require a single-compartment rotor with a combination of high-pressure side and low-pressure side. The rotor material in which the high-pressure side and the low-pressure side are integrated, that is, the high-low pressure integrated rotor material needs to be a material having both excellent high temperature strength on the high pressure side and excellent room temperature strength and toughness on the low pressure side. As a material that meets this requirement, there is a 2.1 / 4 CrMoV-based low alloy steel (for example, JP-A-50-14527 and JP-A-60-245772).

In recent years, from the viewpoint of effective use of fossil fuels, a combined power generation plant combining a gas turbine, an exhaust gas boiler, and a steam turbine having high thermal efficiency has been noticed and added. From the viewpoint of high efficiency, the gas temperature at the gas turbine inlet has become extremely high, and at present, plants of 1350 ° C class have also appeared, and the steam temperature on the high-pressure side of the steam turbine is also considered to be higher than the conventional 540 ° C class. Has been done. However, the conventional 2/4 CrMoV system high / low pressure integrated rotor material lacks high temperature strength, while the 12% Cr system rotor material is satisfactory in strength, but is costly and economically ineffective.

[0005]

As a high / low pressure integrated rotor material for a steam turbine, 2.1 / 4CrMoV steel (Japanese Patent Laid-Open No. 50-14527), 2.1 / 4CrMoVNb is used.
Steel (Japanese Patent Laid-Open No. 60-245772) is available, and it can handle steam temperatures up to 540 ° C. on the high-pressure side. However, in order to improve the thermal efficiency of the combined plant, there is a demand to raise the steam temperature on the high-pressure side to 565 ° C class. For such usage conditions, the above-mentioned 2.1 / 4CrMoV steel,
The 2/4 CrMoVNb steel has a problem in that it lacks high-temperature strength. These steels obtain high temperature strength by precipitation hardening by fine V or VNb carbides and solid solution strengthening of Mo. However, when used at high temperature, Mo and Cr carbides precipitate and at high temperatures such as 565 ° C these carbides are precipitated. Is significantly coarsened, and the contribution of Mo to solid solution strengthening is reduced, resulting in a large decrease in high-temperature strength on a high-temperature long-term side.

In view of the above-mentioned state of the art, the present invention has excellent high temperature strength in the high pressure portion and excellent room temperature strength and toughness in the low pressure portion due to the optimum alloy composition, and can be sufficiently used in the high temperature region of 565 ° C. It is intended to provide a high strength / high toughness low alloy steel.

[0007]

That is, the present invention provides Mo
Instead of solid solution strengthening by, the atomic radius is larger than Mo,
The solid solution strengthening by W having a slow diffusion rate prevents the deterioration of the high temperature strength on the high temperature and long time side, and limits the addition amount of Mn, Si and Ni which tends to lower the high temperature strength. This is addressed by melting low-alloy steel having extremely few impurity elements by adopting vacuum carbon deoxidation and ladle refining, and the present invention (1) is a weight ratio of C:
0.1 to 0.3%, Si: 0.05% or less, Mn: 0.
1% or less, Ni: 0.1 to 1.5%, Cr: 0.5 to 3
%, Mo: 0.05 to 0.5%, V: 0.1 to 0.35
%, Nb: 0.01 to 0.15%, W: 0.5 to 2%,
B: High strength, containing 0.001 to 0.01%, the balance being Fe and inevitable impurity elements.
High toughness low alloy steel, (2) In the high strength / high toughness low alloy steel of (1) above, Mo equivalent = Mo + 1 / 2W (wt%)
Is 0.7 to 1.4, and the high strength / high toughness low alloy steel and (3) the high strength / high toughness low alloy steel of (1) or (2) above, in a weight ratio, A high-strength, high-toughness low-alloy steel characterized in that Co is added by 2% or less and Ti is added by 0.05% or less.

The high-strength, high-toughness low-alloy steel of the present invention is a low-alloy steel having the alloy composition shown in (1) to (3) above.
It can be obtained by vacuum carbon deoxidation, ladle refining, or melting using electroslag melting method, and wrought as in the prior art and optimal heat treatment for obtaining the target strength from the alloy composition.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The composition of the low alloy steel of the present invention and the reasons for limiting the content thereof will be described below. In the following description,% means% by weight.

C (carbon): C improves hardenability,
It is an essential element for ensuring room temperature strength and toughness, and also forms carbides such as Nb, V and Cr and contributes to high temperature strength. In order to obtain the room temperature strength, high temperature strength and toughness necessary for the present invention, 0.1% or more is necessary. However, if added in a too large amount, the toughness is adversely affected and the workability deteriorates. It was set to 1 to 0.3%.

Si (Si): Si is an element effective as a deoxidizing agent for molten steel. However, when a large amount of Si is added, SiO ₂ which is a product of deoxidation remains in the steel, impairing the cleanliness of the steel and lowering the toughness. Further, since Si promotes temper embrittlement during use at high temperatures, Si is set to a necessary minimum.
In the present invention steel, since the vacuum carbon deoxidizing method is applied as the deoxidizing method, the Si content was set to 0.05% or less. This will improve the cleanliness of steel, prevent temper embrittlement, and improve toughness.

Mn (manganese): Mn is deoxidation of molten steel
It is an element effective as a desulfurizing agent, and also effective in improving hardenability and strength. However, if too much is added, the toughness and ductility are impaired. Since the vacuum carbon deoxidizing method and ladle refining are used in the steel of the present invention, a small amount of Mn is sufficient.
The maximum content was 0.1%.

Ni (Nickel): Ni is an element effective for improving the hardenability of steel and enhancing the strength and toughness at room temperature, and is an essential element particularly in large forged products such as rotor materials. However, if too much Ni is added, the high temperature strength (creep rupture strength) is impaired and temper brittleness is promoted, so the content was made 0.1 to 1.5%.

Cr (Chromium): Cr is the most important element as an additive element in a normal low alloy steel for rotor materials. C
Addition of r improves corrosion resistance and oxidation resistance, improves hardenability, and improves tensile properties at room temperature. Also Cr
Is an element effective for improving high temperature strength such as creep strength and creep rupture strength. However, the amount of Cr added is 3%
If it exceeds, the toughness is improved, but the high temperature strength tends to be rather lowered. Therefore, the amount of Cr is set to 0.5 to 3%.

Mo (Molybdenum): Like Cr, Mo is an important element as an additive element of ordinary low alloy steel for rotor materials. Addition of Mo to steel improves hardenability, is effective for increasing room temperature strength, and has an effect of preventing temper embrittlement. Further, Mo is an element effective as a solid solution strengthening element for improving high temperature strength such as creep rupture strength by further forming carbides. To obtain these sufficient effects, 0.05% or more is necessary. Further, Mo has a remarkable effect when added together with W, and it is necessary to consider the amount of W added. However, if too much Mo is added, those effects are saturated and the toughness is adversely affected. Further, since Mo is an expensive element, if too much is added, the cost will increase, so 0.5% or less was made.

V (vanadium): V mainly combines with C to form fine VC carbides, which contributes to improvement of high temperature strength such as creep rupture strength. Further, the addition of V is effective for making the crystal grains finer, and is effective for improving the room temperature strength and toughness. However, if added too much, it is rather harmful to high temperature strength and toughness, so the content was made 0.1 to 0.35%.

Nb (niobium): Similar to V, Nb is effective for refining crystal grains, and is effective for improving room temperature strength and toughness. It also forms fine NbC carbides and is effective in improving high temperature strength such as creep rupture strength. However, if added too much, the toughness is impaired, so its content is 0.0
It was set to 1 to 0.15%.

W (tungsten): W is the same as Mo.
Improves high temperature strength such as creep rupture strength by solid solution strengthening and generation of fine carbide. Conventionally, C mainly composed of Mo
r-Mo steel was generally used, but since W has a larger atomic radius and a smaller diffusion rate than Mo, it has a larger effect of increasing creep strength at a high temperature of 550 ° C or higher for a long time.
To obtain this effect, 0.5% or more of W is required, but if added in too large an amount, large-scale products may cause undesirable phenomena such as solidification segregation, and W is an expensive element.
The content was 0.5 to 2%. When Mo and W are added in combination, these effects are remarkable, but the added amount (Mo equivalent) Mo + 1 / 2W is optimally 0.7 to 1.4.

B (Boron): B dissolves in an extremely small amount of grain boundaries to improve the hardenability of steel and disperses and stabilizes carbides, thus contributing to the improvement of long-term creep rupture strength. This effect is hard to be recognized if it is less than 0.001%, and if it exceeds 0.01%, the forgeability is significantly impaired, so the content was made 0.001 to 0.01%.

Co (Cobalt): Co increases the solid solubility limit of carbide forming elements (Nb, V, Mo, etc.), is effective for uniform precipitation of these elements, and increases the temper softening resistance, thus improving the high temperature strength. Contribute to. However, if too much is added, the hardenability of the steel is impaired and Co is an expensive element, so the addition amount was made 2% or less.

Ti (Titanium): Ti combines with C and N to form Ti (C, N). In particular, the bond with N is strong, and addition of a trace amount of Ti is effective for fixing N. In particular, it greatly contributes to the improvement of the high temperature strength of the B-added steel and the improvement of the toughness by reducing the solid solution N. If too much is added, coarse T
Since i (C, N) is formed and the strength and toughness are significantly impaired, the content is made 0.05% or less.

Others: Other incidental impurities other than Fe that are not included in the above, such as P and S, are unavoidable as raw materials for steelmaking, but it is desirable that these are as low as possible. However, since careful selection of raw materials leads to high product cost, P is 0.015% or less, S
Is 0.01% or less. As other impurity elements,
There are Cu, Al, Sn, Sb, Pb, As and the like.

[0023]

EXAMPLES The effects of the high-strength, high-toughness low-alloy steel of the present invention will be clarified below by giving specific examples of the present invention. The low alloy steels (A to E) of the present invention having the chemical compositions shown in Table 1 were melted in a laboratory-scale vacuum melting furnace to produce a steel ingot of 50 kg.

[0024]

[Table 1]

These steel ingots are heated and forged in a rotor of an actual machine (upset: 1 / 2.8U (forging steel ingot material to a height of 1 / 2.8)), forging: 3.7S (upset). The thickened one was forged to a length of 3.7 times)}, and a small forged material was manufactured. After that, the forged material is subjected to a preliminary heat treatment (for example, 1050 ° C. air cooling and 720 ° C.
Air-cooled). Next, in this high-pressure low-pressure integrated rotor material manufacturing, the forged material was forced air-cooled for maximum diameter of 1600 mm, and the maximum diameter of low-pressure area was 2000 mm for water quenching. The heat treatment was performed. That is, as a heat treatment of the portion corresponding to the high pressure portion, after heating at 950 ° C. to completely austenite, the quenching cooling rate (average of 950 ° C. to 300 ° C.) of the central portion of the high pressure portion: about 25 ° C./hr (heat treatment I), Quenching cooling rate (950 ° C ~
(Average of 300 ° C.): Approximately 1600 ° C./hr (heat treatment II), followed by heat treatment I and heat treatment II, followed by tempering in the temperature range of 650 to 700 ° C., and room temperature proof stress required for the high pressure part: 70 kg / It was adjusted to be mm ² . next,
As a heat treatment condition for the portion corresponding to the low pressure part, after heating at 900 ° C. to completely austenite, the quenching cooling rate (average of 900 ° C. to 300 ° C.) at the center of the low pressure part: about 50 ° C./h
r (heat treatment III), quenching cooling rate (9
(Average of 00 ° C to 300 ° C): 600 to 650 after quenching at two cooling rates of about 1600 ° C / hr (heat treatment IV)
Tempering was carried out in the temperature range of ℃, and adjusted so that the yield strength at room temperature required for the low pressure part was 75 kg / mm ² . The same heat treatment as above was performed on the comparative materials F and G.

A room temperature tensile test and a Charpy impact test were carried out on the test material subjected to the heat treatment (quenching and tempering) of the central part and outer peripheral part of the high pressure part and the central part and outer peripheral part of the low pressure part. Further, a creep rupture test was carried out on the test material which had been subjected to heat treatment (quenching and tempering) in the central portion and the outer peripheral portion of the high pressure portion. For the room temperature tensile test, a test piece having a parallel part diameter of 8 mm and a gauge length of 28 mm was used. For the Charpy impact test, 10 × 10 × 55 mm, 2 mm V notch Charpy impact test piece was used, and the ductile fracture surface ratio of the test piece after the test was 50.
The temperature at which the percentage (% is called 50% FATT) was obtained. For creep rupture test, parallel part diameter: 6.4 mm × gauge length:
A 25.6 mm test piece was used. Larson Miller parameter {LMP = (T + 273) (20 + log
t), T: test temperature, t: test time},
The creep rupture strength at 600 ° C. × 10 ⁴ hr was determined. Table 2 shows the test results.

[0027]

[Table 2]

[0028]

[Table 3]

The tensile properties of the material of the present invention and the comparative material are as follows: proof stress at high pressure part 70 kg / mm ² (I, II), proof stress at low pressure part 75
It can be seen that the value is almost adjusted to kg / mm ² (III, IV). Next, 50% FATT has no significant difference between the material of the present invention and the comparative material, and is +40 to + in the central portion (heat treatment I) of the high pressure portion.
+3 to 12 at + 52 ℃, high temperature area (heat treatment II)
+29 to +35 at the center of low pressure part (heat treatment III)
The temperature was -3 to + 5 ° C in the outer peripheral portion (heat treatment IV) of the low pressure portion.

The creep rupture strength at 600 ° C. × 10 ⁴ hr was 9.2 kg / mm at the outer peripheral part of the high pressure part (heat treatment II) of the comparative material.
^2, whereas a 11.0 kg / mm ^2, it the present invention material is 13.1~14.7kg / mm ^2, it can be seen that significantly creep rupture strength is high. As described above, it was verified that the material of the present invention has high high temperature strength (creep rupture strength) without sacrificing toughness.

[0031]

EFFECTS OF THE INVENTION As has been clarified in the above-mentioned embodiments, the material of the present invention is extremely excellent in creep rupture strength at high temperature without sacrificing toughness, and can be used in a higher temperature range for a single turbine turbine casing. It provides a high-low pressure integrated rotor material, and contributes greatly to reducing the plant cost and improving the thermal efficiency of the plant.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Ryutaro Magoshi 1-1-1, Niihama, Arai-cho, Takasago, Hyogo Prefecture Mitsubishi Heavy Industries, Ltd. Takasago Plant (72) Ichiro Tsuji 2-chome, Niihama, Arai-cho, Takasago, Hyogo Prefecture No. 1 Mitsubishi Heavy Industries, Ltd. Takasago Plant

Claims (3)

[Claims] 1. A weight ratio of C: 0.1 to 0.3%, S
i: 0.05% or less, Mn: 0.1% or less, Ni: 0.
1-1.5%, Cr: 0.5-3%, Mo: 0.05-
0.5%, V: 0.1 to 0.35%, Nb: 0.01 to
0.15%, W: 0.5 to 2%, B: 0.001 to 0.
A high-strength, high-toughness low-alloy steel characterized by containing 01% and the balance being Fe and inevitable impurity elements. 2. The high-strength, high-toughness low-alloy steel according to claim 1, wherein Mo equivalent = Mo + 1/2 W (wt%) is 0.
High strength / high toughness low alloy steel characterized by being 7 to 1.4. 3. The high-strength / high-toughness low-alloy steel according to claim 1, further comprising 2% by weight or less of Co,
A high-strength, high-toughness low-alloy steel characterized by containing Ti in an amount of 0.05% or less.