JPH07118812A - Heat-resistant cast steel turbine casting and its production - Google Patents

Abstract

PURPOSE:To produce a heat-resistant cast steel casing high in creep fracture strength at a high temp., having good weldability and used for a supercritical turbine in which the main steam temp. and pressure are regulated to 621 deg.C and 250kgf/cm<2>, respectively. CONSTITUTION:This casing is constituted of a heat-resistant cast steel contg., by weight, 0.06 to 0.16% C, <0.4% Si, <1% Mn, 8 to 12% Cr, 0.2 to 0.9% Ni, 0.05 to 0.3% V, 0.01 to 0.15% Nb, 0.01 to 0.08% N, <1% Mo, >1 to <3% W, <0.0027% B, and the balance substantial Fe. The ratio of Ni/W is preferably regulated to 0.25 to 0.75, furthermore, at least one kind of <=0.15% Ta and <=0.1% Zr may be added, and moreover, the Cr equivalent calculated by the following formula is preferably regulated to 4 to 10: the Cr equivalent = Cr+6Si+4Mo+1.5W+11V+5Nb-40C-30N-30B-2Mn-4Ni-2Co.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel heat-resistant cast steel turbine casing and a method for producing the same, and in particular, it has a high creep rupture strength at 621 ° C. or higher, good weldability, and main steam temperature and pressure. 621 ℃ and 250 kgf / cm ² respectively
The present invention relates to a turbine casing made of heat-resistant cast steel, which is suitable for the high-pressure and medium-pressure inner casings of the ultra-supercritical turbine, the main steam stop valve and the control valve casing.

[0002]

2. Description of the Related Art Conventional steam turbines have a maximum steam temperature of 566.
℃, maximum steam pressure 246 kgf / cm ² . As this casing material, 1Cr-1Mo-1 / 4V low alloy cast steel or 11Cr-1
Mo-V-Nb-N cast steel is used.

However, from the viewpoint of depletion of fossil fuels such as oil and coal and energy saving, there is a demand for higher efficiency of thermal power plants. The most effective means to raise the power generation efficiency is to raise the steam temperature of the steam turbine. As a material for these high-efficiency turbines, the strength of the current casing material is insufficient, and a material having higher strength than this is required.
However, any of the above-mentioned cast steels lacks high-temperature strength as a high-temperature steam turbine casing having a steam temperature of 621 ° C or higher.

[0004]

The austenitic cast steel disclosed in Japanese Patent Laid-Open No. 61-23749 developed by the present inventors is known as a material having higher high temperature strength than the above-mentioned conventional casing material. ing. However, although these alloys are excellent in high temperature creep rupture strength, they have a problem that a large thermal stress is generated when the turbine is started and stopped because of high cost and large thermal expansion coefficient.

An object of the present invention is to provide a ferritic heat-resistant cast steel turbine casing having a coefficient of thermal expansion equivalent to that of conventional materials, high creep rupture strength at 621 ° C. or higher, and good weldability, and a method for producing the same. To provide.

[0006]

In order to achieve the above object, the heat-resistant cast steel turbine casing of the present invention has a weight ratio of C 0.06 to 0.16%, Si less than 0.4%, Mn less than 1%, Cr 8
~ 12%, Ni 0.2 ~ 0.9%, V 0.05 ~ 0.3%, Nb 0.01 ~
0.15%, N 0.01-0.08%, Mo less than 1%, W over 1%
It is characterized by containing less than 3% and B less than 0.0027%, and the balance being composed of heat-resistant cast steel consisting of Fe and unavoidable impurities. And it is preferable that the Ni / W ratio is 0.25 to 0.75.

Another heat-resistant cast steel turbine casing according to the present invention has a weight ratio of C 0.09 to 0.14%, Si less than 0.3%, and M.
n 0.40 to 0.70%, Cr 8 to 10%, Ni 0.4 to 0.7%, V 0.
15-0.25%, Nb 0.04-0.08%, N 0.02-0.06%, Mo
It is characterized by containing 0.40 to 0.80%, W 1.4 to 1.9%, B 0.001 to 0.0025%, and the balance being heat-resistant cast steel composed of Fe and unavoidable impurities.

It is preferable that the composition of each heat-resistant cast steel turbine casing of the present invention further contains at least one of Ta 0.15% or less and Zr 0.1% or less. Further, the Cr equivalent calculated by the following formula is preferably 4-10.

Cr equivalent = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb-40C-30N-30B-2Mn-4Ni-2Co (1) Further, the heat-resistant cast steel constituting each heat-resistant cast steel turbine casing of the present invention is 625 ℃, 10 ⁵ h creep rupture strength 9 kgf
/ mm ² or more, room temperature impact absorption energy of 1 kgf-m or more, and good weldability. Further, to secure higher reliability, 625 ° C., a 10 ⁵ h creep rupture strength 10
kgf / mm ² or more, it is preferable that the impact absorption energy at room temperature 2 kgf-m or more.

The present invention is characterized in that the method for producing a heat-resistant cast steel turbine casing is characterized in that an alloy raw material having the above-mentioned heat-resistant cast steel casing material as a target composition is melted in an electric furnace, and after ladle refining, it is cast into a sand mold. To do. Then, after the cast molding, it is preferable to perform a normalizing heat treatment of annealing at 1000 to 1150 ° C., heating to 1000 to 1100 ° C. and quenching, and then performing two tempering at 550 to 750 ° C. and 670 to 770 ° C.

[0011]

The composition of the heat-resistant cast steel according to the present invention is limited as follows. C is an element necessary for 0.06% or more to obtain high tensile strength, but if it exceeds 0.16%, the metal structure becomes unstable when it is exposed to high temperature for a long time, and the creep rupture strength for a long time. Is limited to 0.06 to 0.16%. In particular, 0.09 to 0.14% is preferable.

N is effective in improving creep rupture strength and preventing the formation of harmful δ ferrite structure (reducing toughness and fatigue strength), but if less than 0.001%, the effect is sufficient and exceeds 0.08%. And toughness and creep rupture strength. In particular, 0.02 to 0.06% is preferable.

Mn is added as a deoxidizer,
The effect is achieved with a small amount of addition, and a large amount of addition exceeding 1% lowers the creep rupture strength. Especially 0.4-0.7%
Is preferred.

Although Si is also added as a deoxidizing agent, according to steelmaking technology such as vacuum carbon deoxidizing method, Si is added.
No deoxidation is necessary. Further, by lowering Si, there is an effect of preventing harmful δ ferrite structure formation. Therefore,
If added, it should be kept below 0.4%, especially
It is preferably less than 0.3%.

V has the effect of increasing the creep rupture strength. If it is less than 0.05%, its effect is insufficient, and if it exceeds 0.3%, δ ferrite is formed to reduce the fatigue strength. Particularly, 0.15 to 0.25% is preferable.

Nb is a very effective element for increasing the high temperature strength, but if added in a too large amount, coarse eutectic Nb carbides are produced especially in a large steel ingot, which rather lowers the strength or increases the fatigue strength. It causes the precipitation of δ-ferrite, which lowers the temperature, so it must be kept to 0.15% or less.
If the Nb content is less than 0.01%, the effect is insufficient. Particularly in the case of a large steel ingot, 0.02 to 0.1% is preferable, and 0.04 to 0.08% is more preferable.

Ni is a very effective element for increasing the toughness and preventing the formation of δ ferrite, but 0.2%
If it is less than 0.9%, the effect is not sufficient, and if it exceeds 0.9%, the creep rupture strength is lowered, which is not preferable. In particular
0.4 to 0.7% is preferable.

Cr has the effect of improving high strength and high temperature oxidation. When it exceeds 12%, it causes harmful formation of δ-ferrite structure, and when it is less than 8%, the oxidation resistance to high temperature and high pressure steam becomes insufficient. Further, addition of Cr has an effect of increasing creep rupture strength, but excessive addition causes harmful formation of δ ferrite structure and deterioration of toughness. Especially 8.0
~ 10%, more preferably 8.5-9.5%.

W has the effect of significantly increasing the high temperature long-term strength. If less than 1% W, the effect is insufficient as a heat resistant steel used at 621 to 650 ° C. Further, if W exceeds 3%, the toughness becomes low. 1.2-2.0% is preferable, especially 1.4
~ 1.8% is preferred.

Mo is added to improve the high temperature strength. However, in the case where the cast steel of the present invention contains more than 1% W, addition of 1% or more of Mo lowers the toughness and fatigue strength, so it is limited to less than 1%. Particularly, 0.4 to 0.8% is preferable, and 0.55 to 0.70% is more preferable.

In the present invention, the important point is the adjustment of the Ni / W ratio. By adjusting the Ni / W ratio to 0.25 to 0.75, 625 ° C required for the super-supercritical turbine high pressure and medium pressure inner casing of 621 ° C, 250 kgf / cm ² or more, main steam stop valve and control valve casing, 625 ° C. , 10 ⁵ h Creep rupture strength 9 kgf / mm ² or more, room temperature shock absorption energy 1 kgf-m
The above heat-resistant cast steel casing material is obtained.

The addition of Ta and Zr has the effect of enhancing the low temperature toughness, and a sufficient effect can be obtained by adding Ta 0.15% or less and Zr 0.1% or less alone or in combination. When Ta is added by 0.1% or more, the addition of Nb can be omitted.

Since the high temperature creep rupture strength and the low temperature toughness of the heat-resistant cast steel casing material of the present invention are lowered when the δ ferrite structure is mixed, the structure is preferably a uniform tempered martensite structure. In order to obtain a tempered martensite structure, the Cr equivalent calculated by the formula (1) must be 10 or less by adjusting the composition. If the Cr equivalent is too low, the high temperature creep rupture strength will decrease, so it must be 4 or more. Especially, Cr equivalent 6-9
Is preferred.

The addition of B remarkably enhances the creep rupture strength at high temperature (621 ° C. or higher). If the B content exceeds 0.0028%, the weldability deteriorates, so the upper limit is limited to 0.0028%. The B content of the large casing is preferably 0.0005 to 0.0025%,
In particular, 0.001 to 0.002% is preferable.

Since the turbine casing is exposed to high-pressure steam at 621 ° C. or higher, high stress due to internal pressure acts. Therefore, the casing material should be
Creep rupture strength of 9 kgf / mm ² or more at 625 ° C for 10 ⁵ h is required. At the time of starting the turbine, thermal stress acts when the metal temperature is low.
Room temperature shock absorption energy of gf-m or more is required. Especially, in order to ensure higher reliability, 625 ℃, 10 ⁵ h
It is preferable that the creep rupture strength is 10 kgf / mm ² or more and the room temperature impact absorption energy is 2 kgf-m or more.

To produce a casing with few defects,
Since the ingot weighs around 50 tons, it requires a high level of manufacturing technology. The ferritic heat-resistant cast steel casing material according to the present invention described above is prepared by melting the alloy raw material having the target composition of the heat-resistant cast steel in an electric furnace, ladle refining, and casting it into a sand mold to produce a sound product. it can. By performing sufficient refining and deoxidation before casting, it is possible to reduce casting defects such as shrinkage cavities.

Also, the molded heat-resistant cast steel is heated to 1000 to 1150 ° C.
After anneal heat treatment at 60 ° C., it is heated to 1000 to 1100 ° C. and rapidly cooled. Normal heat treatment at 550 to 750 ° C. and 670 to 770 ° C.
Bottle casings can be manufactured. The annealing and normalizing temperature is 10
Carbonitride cannot be sufficiently dissolved at 00 ° C or lower,
If it is too high, it may cause coarsening of crystal grains. Also, 2
The temper tempering completely decomposes the retained austenite,
A uniform tempered martensite structure can be obtained. By manufacturing with the above manufacturing method, 9 kgf / mm ² or more
A creep rupture strength of 625 ° C for 10 ⁵ hours and room temperature impact absorption energy of 1 kgf-m or more are obtained, and a steam turbine casing usable in steam of 621 ° C or more can be obtained.

FIG. 1 is a sectional view of a steam turbine according to the present invention. Steam enters from the main steam pipe 1, is injected in a predetermined direction by the stationary blades 3 attached to the inner casing 2, and the injection causes the moving blades 5 attached to the rotor shaft 4 to rotate. The steam that has worked passes between the outer casing 6 and the inner casing 2 and is discharged from the cooling steam outlet 7. Further, the discharged steam is sent to a steam turbine operating with a lower pressure steam, or sent to a boiler to be reheated and sent to the steam turbine again. The start / stop of the turbine and the output adjustment are performed by adjusting the flow rate of the steam sent from the boiler with the main steam stop valve 9 and the regulator valve 10 shown in FIG. The casings of the main steam stop valve 9 and the regulator valve 10 are welded to each other 11
Are joined by.

Hereinafter, the inner casing according to the present invention will be 9
Cr ferritic cast steel, ferritic 12Cr forged steel similar to the steel according to the present invention is used for the rotor shaft, and Cr-Mo-V cast steel is used for the outer casing.

Particularly, in the present invention, 621 ° C., 250 kgf /
It is suitable for ultra-supercritical turbine high-pressure and intermediate-pressure inner casings of cm ² or more, main steam stop valve casings, and control valve casings.

[0031]

EXAMPLES Examples of the present invention will be described below. [Example 1] With respect to the ferritic heat-resistant cast steel according to the present invention and a comparative material, a weldability test, an impact test at room temperature, a high temperature creep test (625 ° C) and the like were performed.

Tables 1 and 2 show the chemical compositions of the samples used in the above various tests. Assuming a thick portion of a large casing, a sample was melted by 200 kg in a high frequency induction melting furnace and cast into a sand mold having a maximum thickness of 200 mm, a width of 380 mm and a height of 440 mm to produce an ingot. Samples No. 1 to 10 shown in Table 1 are comparative materials, and samples No. 11 to 14 shown in Table 2 are materials of the present invention. Among them, samples No. 1 and No. 2 are Cr-Mo-V cast steel and 11Cr-1Mo-V-Nb-N cast steel used in the current turbine.

[0033]

[Table 1]

[0034]

[Table 2]

Each sample was subjected to furnace cooling annealing at 1050 ° C. for 8 hours,
Heat treatment (normalization / tempering) was performed under the following conditions assuming the thick part of the large steam turbine casing.

Sample No. 1: 1050 ° C. × 8 h Air cooling 710 ° C. × 7 h Air cooling 710 ° C. × 7 h Furnace cooling Sample No. 2 to No. 14: 1050 ° C. × 8 h Air cooling 720 ° C. × 7 h Air cooling 720 ° C. × 7 h Furnace cooling The weldability was evaluated by a diagonal Y-shaped weld crack test according to JIS Z3158. FIG. 3 shows the shape and size of the test piece. The preheating, inter-pass and post-heating start temperatures were 150 ° C, and the post-heat treatment was 400 ° C x 30 minutes.

Table 3 is for the comparative material, and Table 4 is for the material of the present invention. Tensile properties at room temperature, V-notch Charpy impact absorption energy at 20 ° C. and 10 ⁵ h at 625 ° C., respectively.
The results of creep rupture strength and weld cracking are shown.

[0038]

[Table 3]

[0039]

[Table 4]

Material of the present invention to which appropriate amounts of B, Mo and W are added
The creep rupture strength and impact absorption energy of Nos. 11 to 14 are the characteristics required for the ultra-supercritical turbine rotor (625
℃, 10 ⁵ h Strength ≧ 9kgf / mm ² , 20 ℃ Impact absorption energy ≧ 1k
gm) is fully satisfied. Further, the toughness at 20 ° C. of Sample Nos. 13 and 14 to which Ta and Zr are added is considerably excellent. Further, the material of the present invention has no weld cracks and has good weldability. FIG. 4 shows the relationship between the amount of B and weld cracking.
Weld cracks occurred in Comparative Materials No. 3 and No. 4, which contained a large amount of B. Looking at the effects of Mo on the mechanical properties in Tables 3 and 4, Comparative Material No. 6 containing 1.18% Mo has a high creep rupture strength but a low impact value and cannot satisfy the required toughness. . On the other hand, the comparative material No. 5 containing 0.11% of Mo has high toughness but low creep rupture strength, and cannot satisfy the required strength (Fig. 5).

FIG. 6 shows the effect of Ni / W ratio on mechanical properties. If the Ni / W ratio is increased too much, the creep rupture strength will decrease. On the contrary, if the Ni / W ratio is too low, the room-temperature impact absorption energy becomes low. Ni / W ratio of 0.25 to 0.7
By adjusting to 5, the temperature is 621 ℃, the pressure of 250kgf / cm ² or more ultra-supercritical turbine high pressure and medium pressure inner casing, as well as the main steam stop valve and control valve casing, required.
A heat-resistant cast steel casing material having a creep rupture strength of 9 kgf / mm ² or more and a room temperature impact absorption energy of 1 kgf-m or more can be obtained at 625 ° C for 10 ⁵ h. Particularly, by adjusting the Ni / W ratio to 0.25 to 0.75, an excellent heat-resistant cast steel casing material having a creep rupture strength of 10 kgf / mm ² or more at 625 ° C. for 10 ⁵ h and a room temperature impact absorption energy of 2 kgf-m or more can be obtained.

Example 2 One ton of an alloy raw material having a target composition of the heat-resistant cast steel according to the present invention was melted in an electric furnace, and after ladle refining, cast into a sand mold. This formed cast steel is held at 1050 ℃ × 8h, and after furnace annealing annealing heat treatment, 1050 ℃ × 8h
Held at 730 ° C. for 8 hours and 730 ° C. for 8 hours. As a result of cutting and investigating this prototype casing, it is necessary to sufficiently satisfy the characteristics required for an ultra-supercritical turbine casing (625 ° C, 10 ⁵ h creep strength ≧ 9 kgf / mm ² , 20 ° C shock absorption energy ≧ 1 kg-m). It was confirmed that the weldability was good. This proves that a steam turbine casing that can be used in steam at 621 ° C or higher can be manufactured.

[0043]

EFFECTS OF THE INVENTION According to the present invention, a ferritic heat-resistant cast steel having a high 625 ° C. creep rupture strength and a high room temperature toughness can be obtained. The member can be made of ferritic heat-resistant cast steel (material of the present invention) instead of the conventional austenitic heat-resistant cast steel.

By using the heat-resistant cast steel according to the present invention in the turbine casing instead of the conventional heat-resistant austenitic cast steel, the material cost can be remarkably reduced. Further, since the ferritic heat-resistant cast steel according to the present invention has a smaller coefficient of thermal expansion than the austenitic heat-resistant cast steel, it has advantages that the turbine can be rapidly started easily and that it is less susceptible to thermal fatigue damage.

[Brief description of drawings]

FIG. 1 is a sectional view of a steam turbine including a heat-resistant cast steel turbine casing of the present invention as a constituent element.

FIG. 2 is a view showing casings of a main steam stop valve and a control valve.

FIG. 3 is a diagram showing the shape of a weld crack test piece of heat resistant cast steel according to the present invention.

FIG. 4 is a diagram showing the relationship between the amount of B and welding cracks in the material of the present invention.

FIG. 5 is a diagram showing the effect of Mo on the mechanical properties of the material of the present invention.

FIG. 6 shows Ni / affecting mechanical properties of the material of the present invention.
It is a figure which shows the influence of W ratio.

[Explanation of symbols]

 1 Main Steam Pipe 2 Inner Casing 3 Stator Blade 4 Rotor Shaft 5 Moving Blade 6 Outer Casing 7 Cooling Steam Outlet 9 Main Steam Stop Valve 10 Control Valve

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location F03B 11/02 7504-3H (72) Inventor Shigeyoshi Nakamura 7-1 Omika-cho, Hitachi-shi, Ibaraki Stock ceremony Hitachi, Ltd., Hitachi Research Laboratory (72) Inventor, Hiroshi Fukui, 7-1, 1-1 Omika-cho, Hitachi, Hitachi Within Hitachi, Ltd.

Claims (8)

[Claims] 1. A weight ratio of C 0.06 to 0.16%, Si less than 0.4%, Mn less than 1%, Cr 8 to 12%, Ni 0.2 to 0.9%, V
0.05 to 0.3%, Nb 0.01 to 0.15%, N 0.01 to 0.08%, M
o Heat-resistant cast steel turbine casing, characterized in that it contains less than 1%, more than W 1% and less than 3%, and B less than 0.0027%, and the balance is composed of heat-resistant cast steel consisting of Fe and inevitable impurities. 2. The heat-resistant cast steel turbine casing according to claim 1, wherein the ratio Ni / W of the contents of Ni and W in the heat-resistant cast steel is 0.25 to 0.75. 3. A weight ratio of C 0.09 to 0.14%, Si less than 0.3%, Mn 0.40 to 0.70%, Cr 8 to 10%, Ni 0.4 to 0.7.
%, V 0.15 to 0.25%, Nb 0.04 to 0.08%, N 0.02 to 0.0
6%, Mo 0.40 to 0.80%, W 1.4 to 1.9%, B 0.001 to 0.
A heat-resistant cast steel turbine casing, characterized in that the heat-resistant cast steel is composed of heat-resistant cast steel containing Fe and unavoidable impurities in the balance. 4. A heat-resistant cast steel turbine casing according to claim 1, further comprising at least one of Ta 0.15% or less and Zr 0.1% or less. Heat resistant cast steel turbine casing. 5. The heat resistant cast steel has a C calculated by the following equation.
5. The r equivalent is 4 to 10, wherein the r equivalent is 4 to 10.
A heat-resistant cast steel turbine casing according to any one of 1. Cr equivalent = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb-40C-30N-30B-2Mn-4N
i-2Co 6. The heat resistant cast steel has a creep rupture strength of 9 kgf / mm ² or more at 625 ° C. for 10 ⁵ h, and a room temperature impact absorption energy of 1 k.
The heat-resistant cast steel turbine casing according to any one of claims 1 to 5, which has gf-m or more and has good weldability. 7. The heat-resistant cast steel casing material according to claim 1 is used to melt an alloy raw material having a target composition in an electric furnace, and after ladle refining, it is cast into a sand mold. Heat resistant cast steel turbine casing manufacturing method. 8. After the cast molding, a normalizing heat treatment of annealing at 1000 to 1150 ° C., heating to 1000 to 1100 ° C. and quenching is performed, and tempering is performed twice at 550 to 750 ° C. and 670 to 770 ° C. A method for manufacturing a heat-resistant cast steel turbine casing according to claim 7.