JPH0570902A - High cr alloy steel - Google Patents

Abstract

PURPOSE:To provide high Cr alloy steel capable of enhancing creep strength without causing secular embrittlement. CONSTITUTION:This high Cr alloy steel consists of, by weight, 0.1-5-0% Y2O3, 0.05-0.30% C, <=0.3% Si, <=1.0% Mn, 0-3-2-0% Hi, 8.0-13.0% Cr, 0.5-2.0% Mo, 0.1-0.3% V, 0.03-0.30% Nb, 0.01-0.20% N, 0.1-3.0% W and the balance essentially Fe.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high C material suitable as a constituent material for a blade of a steam turbine, a bolt for fastening a casing or the like.
The present invention relates to r alloy steel, and more particularly to high Cr alloy steel with improved creep strength and reduced aging embrittlement.

[0002]

^2. Description of the Related Art Generally, steam used for driving a steam turbine has a maximum temperature of 566 ° C. and a pressure of 246 kgf / cm ²
However, in order to improve the thermal efficiency, it has recently been required to further increase the temperature and pressure.

Such steam conditions are highly dependent on the high temperature strength of the material constituting the turbine, and in order to realize the improvement of the steam conditions, the development of high temperature strength materials has been promoted, for example, 12% Cr. Heat resistant steels such as heat resistant steels have been put to practical use.

[0004]

By the way, the development of such materials is of course very important not only for large parts such as rotors and casings but also for high speed rotating parts such as blades and bolts and pressure resistant parts. Is.

That is, since centrifugal force due to high-speed rotation acts on the blades of the steam turbine, if the high-temperature strength is insufficient, the blades will creep and float from the rotor, and the tips will contact the stationary portion. I may arrive.
In addition, the bolt used to seal the casing is initially given a certain tightening pressure based on the elastic force, but since the steam pressure acting on the casing always acts on the bolt, the bolt deforms due to creep deformation. Then, when the tightening pressure gradually decreases, the casing cannot be hermetically sealed to cause steam leakage, and creep deformation is accumulated, the bolt itself may be broken.

On the other hand, the operating temperature range of the blades and bolts of these steam turbines is 350 to 550 ° C., but if a metal material is used for a long time at such a temperature, an embrittlement phenomenon may occur and the toughness may decrease. is there. This is because the metallographic structure changes due to being heated to a high temperature for a long time, but it is possible that a blade rotating at high speed or a bolt placed under high tensile stress becomes brittle during its use. However, the resistance to impact is weakened, which is not preferable.

As described above, the blades and bolts used in the high temperature part of the steam turbine have excellent creep characteristics,
And a material that does not cause aging embrittlement at high temperature is required,
Conventionally, 12% Cr-based heat-resistant steel has been used as described above.

This 12% Cr heat-resisting steel is generally cheaper than other heat-resisting steels having the same high-temperature strength, and is excellent in vibration damping characteristics indispensable as a blade material. However, there is a problem that the high temperature strength is insufficient to cope with the improvement of the steam condition of the steam turbine.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high Cr alloy steel capable of improving creep strength without causing aging embrittlement.

[0010]

MEANS FOR SOLVING THE PROBLEMS AND ACTION The high Cr alloy steel according to the present invention contains 0.1 to 5.0% by weight of oxide, 0.05 to 0.30% by weight of C, and 0. 3% by weight or less,
Mn is 1.0 wt% or less, Ni is 0.3 to 2.0 wt%, Cr is 8.0 to 13.0 wt%, and Mo is 0.5 to.
2.0% by weight, V 0.1 to 0.3% by weight, Nb 0.
It is characterized in that it is 03 to 0.30% by weight, N is 0.01 to 0.20% by weight, W is 0.1 to 3.0% by weight, and the balance is substantially Fe.

Here, the oxide is used for preventing the movement of dislocations by its dispersion and imparting the creep rupture strength, and it is effective to disperse the oxide finely and uniformly. For that purpose, the addition amount is required to be 0.1% by weight or more. However, if it exceeds 5.0% by weight, the amount of oxide becomes excessive, and on the contrary, the creep rupture strength decreases and the toughness also decreases.

Therefore, the addition amount of the oxide is 0.1-0.1%.
It is set to 5.0% by weight. The preferable addition amount is 0.3.
~ 2.5% by weight.

As the oxide, Y ₂ O ₃ which is stable at high temperature and easily available is preferable. However, similar effects can be expected with other oxides.

Further, in the present invention, C (carbon) is used for stabilizing the austenite phase during quenching and further for forming carbides to enhance the creep rupture strength, and for that purpose, 0.05 wt. % Or more is necessary. However, if the addition amount exceeds 0.30% by weight, the carbides become excessive and the creep rupture strength is rather lowered.

Therefore, the addition amount of C is from 0.05 to
The weight is 0.30. Preferably 0.08-0.20
% By weight.

Further, in the present invention, Si (silicon)
Is added as a deoxidizing agent at the time of melting, but when the addition amount increases, a part thereof remains in the steel as an oxide, which not only adversely affects the toughness, but also the δ ferrite phase and It acts to generate the Laves phase and causes aging embrittlement.

Therefore, the addition amount of Si is set to 0.3% by weight or less. It is preferably 0.1% by weight or less.

Further, in the present invention, Mn (manganese)
Similar to Si, is added as a deoxidizing / desulfurizing agent at the time of dissolution, but if the amount of addition is large, the creep rupture strength and toughness decrease.

Therefore, the addition amount of Mn is set to 1.0% by weight or less. Preferably, it is 0.3 to 0.8% by weight.

Further, in the present invention, Ni (nickel) is used.
Is added as an austenite forming element,
It is effective in stabilizing the austenite phase during quenching and preventing the formation of the δ ferrite phase.
It is necessary to add more than 0.3% by weight. However, if it is added in an amount of more than 2.0% by weight, the creep rupture strength is extremely lowered and the A _c1 transformation temperature is lowered.

Therefore, the amount of Ni added is 0.3 to
It is set to 2.0% by weight. It is preferably 0.5 to 1.5% by weight.

Further, in the present invention, Cr (chrome)
Is an element used to prevent oxidation in a high temperature environment and improve creep rupture strength.
For that purpose, the addition amount of 8.0% by weight or more is necessary. However, if the addition amount exceeds 13.0% by weight, the δ ferrite phase is generated and the toughness and the creep rupture strength decrease.

Therefore, the addition amount of Cr is 8.0 to 1
3.0% by weight. Preferably, it is 9.0 to 12.0% by weight.

In the present invention, Mo (molybdenum) is used for improving creep rupture strength and preventing temper embrittlement. For that purpose, at least 0.5% by weight or more is used. Is required. However, if the addition amount exceeds 2.0% by weight, a δ ferrite phase is generated, and the toughness and creep rupture strength decrease.

Therefore, the addition amount of Mo is 0.5 to
It is set to 2.0% by weight. Preferably, it is 0.7 to 1.5 weight.

In the present invention, V (vanadium)
Is used for improving the creep rupture strength, but for that purpose, the addition amount of 0.1% by weight or more is required. However, if the addition amount exceeds 0.3% by weight, a δ ferrite phase is generated, and the toughness and creep rupture strength decrease.

Therefore, the addition amount of V is 0.1 to 0.
3% by weight. It is preferably 0.15 to 0.27% by weight.

Further, in the present invention, Nb (niobium)
Is used for making the crystal grains finer to improve the ductility and toughness, and at the same time forming carbides or charcoal / nitrides to finely disperse and precipitate them in the matrix to significantly improve the creep rupture strength. For that purpose, the addition amount of at least 0.03% by weight or more is necessary. However, if the addition amount exceeds 0.3% by weight, a δ ferrite phase is generated, and massive carbides or carbon / nitrides are generated, resulting in a decrease in toughness and creep rupture.

Therefore, the amount of Nb added is from 0.03 to
0.30% by weight. Preferably 0.05-0.1
It is 5% by weight.

Further, in the present invention, N (nitrogen) is used for suppressing the formation of ferrite phase and forming carbon / nitride of Nb. % Or more is required. However, if the addition amount exceeds 0.20% by weight, the toughness decreases.

Therefore, the amount of N added is 0.01 to.
0.20% by weight. Preferably 0.03 to 0.0
8% by weight.

Further, in the present invention, W (tungsten) is used to form a solid solution in the matrix like Mo to improve the creep rupture strength.
For that purpose, the addition amount of 0.1% by weight or more is required. However, if the addition amount exceeds 3.0% by weight, the toughness will be reduced.

Therefore, the addition amount of W is 0.1-3.
0% by weight. It is preferably 0.5 to 1.5% by weight.

In the present invention, it is desirable to add B (boron). That is, B has the effects of improving hardenability and creep rupture strength. However, if added in excess of 0.01% by weight, cracking tends to occur during the manufacturing process, so the addition amount of B is set to 0.01% by weight or less. Preferably, 0.
It is 003 to 0.008% by weight.

A preferred method for producing a high Cr alloy steel according to the present invention is that the oxide and alloy powders are mechanically alloyed and mixed, and then subjected to extrusion molding, forging, rolling or HIP treatment, followed by quenching, It is to temper.

More specifically, first, by a vacuum high frequency furnace,
After melting the melting stock of high Cr alloy steel to which alloying elements are added, the melting stock is remelted in a vacuum and a master alloy powder is manufactured by the Ar gas atomizing method. This mother alloy powder and an oxide powder of Y ₂ O ₃ or the like are mixed and mechanically alloyed by using a stirring ball mill. After that, extrusion, forging, rolling or H
IP processing is performed. Further, thereafter, quenching and tempering are performed to adjust the structure of the matrix and the dispersed and precipitated state of carbides, charcoal, nitrides and the like. The alloy steel thus obtained is cut and formed into a desired shape such as a turbine part.

In the case of a blade, a wrought billet is cut into an appropriate size, heated to a temperature of about 1100 to 1200 ° C., and then shaped into a blade by die forging, followed by quenching and tempering. It may then be machined to final dimensions.

[0038]

EXAMPLES Examples of the present invention will be described below.

Table 1 shows Examples 1 to 8 and Comparative Examples 1 to 7.

That is, Examples 1 to 8 and Comparative Example 4 in Table 1
For ~ 7, the raw materials were blended so as to have a predetermined alloy composition, the melting stock of the high Cr alloy steel was melted by the vacuum high frequency furnace, and then the melting stock was remelted in vacuum, and the Ar gas atomization method was used. A mother alloy powder was produced. Next, the mother alloy powders and the Y ₂ O ₃ oxide powders were mixed and mechanically alloyed using a stirring ball mill. The treated powder was filled in a mild steel container, heated at 400 ° C., deaerated in vacuum, and then sealed.
Then, it was sintered by a high temperature isostatic press, taken out from the container after sintering and heated to 1200 ° C. for hammer forging, and forged into a round bar having a diameter of 30 mm.

In Comparative Examples 1 to 3, raw materials were blended so as to have a predetermined alloy composition, melted by high frequency vacuum melting, and then cast into a mold to obtain ingots. After scraping off the surface of this ingot by machining, put it in a heavy oil furnace,
Hammer forging was performed by heating to 1200 ° C., and forged into a round bar having a diameter of 30 mm. In addition, the compositions of Comparative Examples 1 and 2 are Mel, which is a conventionally used 12Cr heat-resistant steel, respectively.
-Trol H-46 and Crucible 422
Comparative Example 3 has the same composition as the alloy steel according to the present invention, but only Si has a high level equivalent to that of the conventional material.

[0042]

[Table 1]

The round bar obtained as described above was cut into lengths capable of collecting test pieces for each test described below, and each was heated and held at 1120 ° C. for 2 hours, and then put into oil at room temperature. Then, it was quenched and subsequently heated in an electric furnace at 590 ° C. for 3 hours to be tempered, and further at 650 ° C. for 3 hours.
Each material that had been subjected to this heat treatment was divided into two groups, and one of them was machined as it was to prepare a test piece, which was subjected to a tensile test, a creep rupture test and a Charpy impact test. On the other side, after heat treatment, after heating in an electric furnace at 600 ° C for 10,000 hours,
A test piece was prepared by machining and a Charpy impact test was performed. The results of these tests are shown in Table 2.

The tensile test was conducted at room temperature, and Table 2 also shows elongation after breakage and drawing. Further, the creep rupture test was conducted under two different conditions by changing the test temperature and stress, and the table shows the time (rupture time) required to reach the rupture under each condition. Furthermore, the Charpy impact test was carried out at a plurality of temperatures ranging from room temperature to 200 ° C. to determine the fracture surface transition temperature.

[0045]

[Table 2]

From the test results shown in Table 2, it can be seen that the alloy steels according to the present example show markedly superior creep rupture properties at both temperatures of 600 ° C. and 650 ° C. as compared with the comparative example. Furthermore, looking at the change in rupture transition temperature after heating at 600 ° C. for 10,000 hours, it was found that 60
In contrast to the increase of about 0 ° C., Examples 1 to 8 have S
Since the i amount is 0.03 to 0.06%, the increase in the fracture surface transition temperature is 8 to 12 ° C., and it can be seen that the aged embrittlement is suppressed to a low level.

[0047]

As described above, the high C according to the present invention
According to the r alloy steel, the creep characteristics can be significantly improved without impairing the ductility, and the aged embrittlement phenomenon can be significantly reduced. Therefore, by adopting the blades and bolts of the steam turbine, the steam condition can be improved, and the turbine can be stably operated.

Claims (1)

[Claims] 1. A weight ratio of oxide is 0.1 to 5.0%,
C is 0.05 to 0.30%, Si is 0.3% or less, Mn
Is 1.0% or less, Ni is 0.3 to 2.0%, and Cr is 8.
0-13.0%, Mo 0.5-2.0%, V 0.1
~ 0.3%, Nb 0.03 to 0.30%, N 0.0
1 to 0.20%, W 0.1 to 3.0%, and the balance being substantially Fe.