JPS60155653A - Iron-base super alloy and its production - Google Patents

Abstract

PURPOSE:To enable production of an iron-base super alloy having excellent high- temp. strength, toughness and high-temp. ductility by melting an Ni-Cr alloy steel in a vacuum atmosphere, decreasing considerably the content of oxygen and hydrogen and adding a specific desulfurizing agent to decrease the content of S. CONSTITUTION:An iron-base super alloy having the compsn. contg. <0.15% C, <2.0% Mn, <1.5% Si, 10-20% Cr, 20-30% Ni, 0.5-3% Mo, 1.5-3% Ti, 0.1- 0.5% Al, 0.002-0.01% B and <0.4% V and contg. <60ppm O2 and <2ppm H2 is melted in a vacuum induction melting device maintained under <=0.05Torr degree of vacuum or partial pressure of air; further 1 or >=2 kinds among Mg, Y, Zn and Zr are added at 0.001-0.05% thereto to fix S in the molten metal thereby preventing precipitation of S at the grain boundary and consequent decrease of ductility. The alloy is suitable as a material for a rotating machine and apparatus for a power plant to be used at 600-650 deg.C.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an iron-based superalloy that is excellent in not only high-temperature strength and toughness but also high-temperature ductility and exhibits notch strengthening on the high-temperature long-term side.
The present invention relates to a material suitable as a rotating equipment material for power generation plan 1 to be used at a temperature of 600 to 650°C, and a manufacturing method thereof.

[Background of the invention]

Conventionally, a steam turbine uses main steam at a temperature of 538 to 566° C., and rotating equipment such as a rotor and rotor blades are formed of Cr-Mo-V and 12Cr@. but,
Due to the decrease in oil resources and the demand for a stable supply of electricity due to this, high-temperature, high-pressure power generation plants with main steam temperatures of 600°C or higher or large-capacity 1! Expansion turbines and the like are being considered for the purpose of electric power plants and energy diversification. All of these power plants have harsh operating conditions. Looking at the rise in operating temperature, Cr-Mo
-V, 12 Cr 6'f4 and other ferrite materials are 5
It has been pointed out that in the temperature range of 50°C or higher, grain boundary climbing becomes noticeable and the creep rupture strength is extremely reduced, which calls for the proposal of a new material.

Currently, austenitic alloys in which Cr and Ni are added to Fe are generally used as materials in the temperature range of 600° C. or higher. Therefore, from the viewpoint of high-temperature strength, high-Ni austenitic alloys are considered promising materials for rotating equipment for power plants used at temperatures of 600° C. or higher.

On the other hand, the above-mentioned materials have so far been tailored to specifications that emphasize strength for relatively short periods of time, such as jet engines, so in order to apply them to equipment materials for power generation plants, they must have creep rupture strength of 103 hours or more and notch strength (high temperature It is necessary to improve the ductility (correlation with ductility).

In particular, in equipment such as blades and rotors where stress concentration areas exist, it is important that the creep rupture strength of the notched material exceeds the creep rupture strength of the smooth material (notch strength) on the long-term side.

[Purpose of the invention]

The purpose of the present invention is to develop an iron-based superalloy that exhibits high-temperature strength such as tensile strength and creep rupture strength, as well as fracture ductility on the high-temperature long-term side and is notch-strengthened, particularly for high-efficiency power generation plants used at 600-650℃. Our purpose is to propose materials for rotating equipment.

[Summary of the invention]

In the present invention, the vacuum degree or air partial pressure is 0.05 torr.
Melted under the following conditions, C0.15% or less by weight, M n
2% or less, Si 1.5% or less, 6110-20%
, N i 20-30%, M o 0, 5-3%, T
i1.5~3%, AQo, 1~0.5%, B O,00
2 to 0.01%, Vo, 4% or less, 060ppm or less,
Limit H2ppm or less, and further 0.001 to 0.05
% Mg, Y, Zn, and Zr are added, and the remainder consists of Fe and unavoidable impurities.

In the present invention, O and H absorbed during melting or from raw materials are
It was found that it is necessary to limit the amount of these gases because they segregate at grain boundaries and reduce fracture ductility, resulting in notch deterioration over long periods of time.

Furthermore, S also segregates at grain boundaries and reduces ductility, but Mg, Y
, Zn, and Zr were found to bind to S and to be effective in fixing S.

Based on the above phenomena, we propose an iron-based superalloy that has excellent high-temperature ductility by limiting the amount of o and H by increasing the vacuum and adding Mg + Ye Zn and Zr.

The reasons for limiting each component are shown below.

C is important because it forms carbides and improves high temperature strength and creep rupture strength. If added in excess of 0.15%, the toughness and weldability will be significantly reduced, so the upper limit is set at 0.15%.

St is an important component as a deoxidizing agent in melt production. However, like C, when added in large amounts, toughness and weldability are reduced, so the upper limit is set at 1.5%.

Like Si, Mn is an important component as a deoxidizing agent in melt production and as a component that further improves hot workability. However, if it exceeds 2%, corrosion resistance and oxidation resistance will decrease, so the upper limit is set at 2%.

Ni is an important component that forms an austenite structure. However, if it is less than 20%, the effect is not sufficient and an unstable austenite structure results. On the other hand, if it exceeds 30%, hot workability will be reduced. Therefore, 6Cr, which needs to be added in a range of 20 to 30%, is an important additive element for improving high temperature strength, corrosion resistance, and oxidation resistance, and in order to obtain this effect, 6Cr should be added in an amount of 10% or more. However, if it exceeds 20%, the weldability will deteriorate and a ferrite phase will form, accelerating embrittlement at high temperatures and long periods of time, so the upper limit is set at 20%.

MO strengthens the austenite base and forms carbides to improve creep rupture strength. 0.5% to 3.0% because this effect cannot be expected if it is less than 5%, and if it is added more than 3.0%, it forms an oxide (Mob) with a low melting point and the low oxidation property becomes very poor. becomes good.

Ti is an element necessary for precipitating the γ' phase (N 1 s (AΩ, Ti)) which is effective in improving high temperature strength and ductility in addition to acting as a deoxidizing agent. If it is less than 1.5%, the effect cannot be fully expected, and if it is more than 3%, the η phase (Ni, Ti), which has no age hardening properties, will precipitate.
Add 3.0%.

Afi combines with Ti and precipitates an intermetallic compound γ' phase.

However, if added in a large amount, the high temperature strength decreases, so the upper limit is set at 0.5%.

B is effective in significantly strengthening grain boundaries and improving high-temperature ductility. However, if it is contained in a large amount, processability is reduced, so the upper limit is set at 0.01%.

V forms precipitates such as VS, VN, and VC.

Among these, VC has age hardening properties and is effective for improving tensile strength and creep rupture strength.

However, an increase in the amount of V adversely affects oxidation resistance. Therefore, the v amount should be 0.4% or less.

The solubility of H in the alloy decreases rapidly at low temperatures, so that these small voids or bubbles cause microcracks in the material. Therefore, an increase in H tends to particularly reduce elongation at break and reduction of area. In recent years, advances in vacuum technology have led to a widespread method of vacuum-treating molten steel to remove hydrogen. However, complete hydrogen removal is impossible because hydrogen is absorbed from the furnace atmosphere, slag, and moisture in the refractories. Therefore, in order to prevent the deterioration of ductility due to H, the amount of H is limited to 2 ppm or less.

0 is reduced by the deoxidation reaction, while some of the various oxides generated in the molten steel by the deoxidation reaction remain as oxide-based nonmetallic inclusions without floating and separating from the molten steel.

These nonmetallic inclusions tend to segregate at grain boundaries, resulting in a significant negative effect on creep rupture ductility. Therefore, the amount of O is 6
It should be limited to 0 ppm or less.

Mg, Y, Zn, Zr, and S have segregation coefficients (0,
98) is one of the elements that is most likely to segregate. Moreover, FeS alone or coexisting with 5in2 becomes a factor in the occurrence of time cracking. This S content can now be reduced with the improvement of desulfurization technology. However, precipitation hardening materials such as the steel of the present invention have relatively low grain boundary strength, and as a result, ductility at high temperatures tends to decrease due to grain boundary segregation of S or sulfide. Therefore, in order to fix S, Mg, Y, Zn, and Zr, which have high binding strength, are added. If any of these elements is less than 0.001%, the effect will not be sufficient, and if it is more than 0.05%, the ductility will be impaired.
Should be limited to ~0.05%.

To limit the amount of O and H to the above component range, the degree of vacuum should be 0.
It should be less than 0.05 torr. For this reason, vacuum melting at a vacuum degree of 10 -1 torr or higher or atmospheric melting at a higher vacuum level is insufficient, and melting by vacuum melting becomes inevitable.

Possible melting methods include vacuum oxygen decarburization and vacuum induction melting. Further, after producing the electrode by such a method, the gas component can be further reduced by performing vacuum arc remelting. Also, this can be replaced by electroslag melting.

[Embodiments of the invention]

FIG. 1 is a sectional view of an embodiment of a steam turbine according to the invention. In the figure, the portion indicated by 12 is the rotor of the present invention, and a plurality of rotor blades 10 are implanted in the rotor. A plurality of stator blades 14 are installed between the moving blades 10, and the rotor 12 passes through an inner casing 16 that fixes the stator blades. A plurality of protrusions 18 are formed on the inner casing 16, and these plurality of protrusions 18 are inserted into recesses of the outer casing 20 that encloses the inner casing and are fixed with bolts or the like.

Further, the external casing 20 rotatably supports both ends of the rotor 12 in the through hole portion 22, and has an outlet 24 formed at the lower left in the figure and an opening 26 at the upper portion.

The main steam flows down in the main steam pipe 30 as shown by the arrow, passes through the nozzle box 28, and flows into the inner casing 16. Thereafter, the moving blades 10 are rotated integrally with the rotor 12 to enter the space between the inner casing 16 and the outer casing 20, and flow out from the outlet 24. Now, assuming that the temperature of the main steam is 650°C and the pressure is 350kgf/cJI, the steam turbine has a temperature of 650°C to 650°C on the rotor blade surface.
554.3℃, pressure 350-199 kg f/c
This is the operating condition for the paddle.

Table 1 shows the chemical composition of the test materials. 1 to 7 of this steel type
is the developed material, and 8 to 11 are comparative materials. The melting method is steel type 1
~3 is vacuum induction melting (VIM), and steel type 4 is electroslag remelting (ESR) after vacuum induction melting (VIM).
), steel grade 5 undergoes electroslag remelting (ESR) after vacuum oxygen decarburization (VOD), and steel grades 6 to 7 undergo vacuum induction melting (VOD).
IM) Post Vacuum Arc Remelt (VAR). Moreover, in the case of steel grades 8 to 11, atmospheric dissolution accounts for the majority.

Further, in each case, solution treatment was performed by heating at 980° C. for 1 to 3 hours, followed by water or oil cooling, followed by aging treatment by heating at 700 to 760° C. for 16 hours, followed by air cooling. The structure consists of γ′ phase precipitated on an austenite base.

Figure 2 shows the relationship between the creep rupture area of each steel type and the amount of O and H at 650°C - 46 kgf/n++n''.This result shows that when the amount of O specified in the present invention is 60 ppm or less,
All steel types that satisfy the H content of 2 ppm or less have an area of area at break of 10% or more. On the other hand, in steel types (steel types 8 to 11) in which either the O or H amount is greater than or equal to this specified value, the fracture area is 10% or less.

When dealing with materials that have stress concentration areas, such as actual machine structures, "notch reinforcement" is an essential condition for the material, in which the creep rupture strength of the notched material is greater than the strength of the smooth material. Generally, when the fracture ductility is large, the amount of creep deformation increases with stress concentration, and stress relaxation occurs in the stress concentration area. As a result, stress concentration decreases, and the high-strength material tends to have high notch strength. In this way, the fracture ductility is a guideline for the notch effect, and the fracture reduction for notch reinforcement is 10%.
It is generally practiced to maintain the

As described above, it can be seen that the material of the present invention satisfies the rupture area of 10% or more, which is one of the criteria for notch reinforcement.

On the other hand, FIG. 3 shows the relationship between the degree of vacuum during melting and the creep rupture area.

From these results, it can be seen that when the degree of vacuum during melting is set to 0.05 torr or less, the rupture area becomes 10% or more in all cases.

Figure 4 shows developed materials (steel types 1, 4, and 5.6) and comparative materials 9.
shows the creep rupture strength of According to the results, the developed material exhibits notch reinforcement in which the strength of the notch material exceeds that of the smooth material within a range up to 104 hours to fracture. On the other hand, Comparative Material 9 becomes weakened in the notch after approximately 3000 hours (smooth material strength≧notch material strength). In this way, the degree of vacuum etc. during melting is set to 0.0.
It can be seen that limiting the amount of O and H to 5 torr or less is effective in improving creep rupture ductility and, as a result, increasing notch strength.

〔Effect of the invention〕

According to the present invention, the degree of vacuum during melting is set to 0.05 torr.
Creep rupture ductility is improved by limiting OX to 60 ppm or less and H amount to 2r'P or less, and further adding Mg, Y, Zn, and Zr, resulting in a good creep rupture strength of 600 to 650 for the notched material. It has become clear that a Fe-based superalloy suitable for rotating equipment for ultra-supercritical pressure turbines can be proposed.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of an ultra-supercritical pressure turbine, Figure 2 is a diagram of the relationship between creep rupture ductility and O and H content, Figure 3 is a diagram of the relationship between degree of vacuum during melting and creep rupture restriction, and Figure 4 is a diagram of the relationship between creep rupture ductility and the amount of O and H. It is a figure showing creep rupture strength. 10... Moving blade, 12... Rotor, 13... Stationary blade,
16...Inner casing, 18...Inner casing (recess)%20...Outer casing, 22...Through hole portion, 24...Outlet, 26...Opening, 28...
Nozzle box, 30...main steam pipe. Agent Patent Attorney Akio Takahashi $2 Figure H amount (prfrL)

Claims (1)

[Claims] 1. A method for producing an iron-based superalloy, characterized in that it is produced by a vacuum melting method in which the degree of vacuum or air partial pressure is 0.05 torr or less. 2. C of 0.15% or less by weight; Mn of 12% or less; 1
．． 5% or less St, 10-20% Cr, 20-30%
of Ni, 0.5-3% Mo, 1.5-3% Ti, 0
．． 1~0.5% AΩ, 0.002~0.01% B,
An iron-based superalloy characterized by containing 0.4% or less of V, further limiting the amounts of O and H to 60 ppn+ and 2 ppm or less, respectively, with the remainder being composed of iron and unavoidable impurities. 3. In the second claim, o, ooi ~ 0
．． An iron-based superalloy characterized by adding 0.5% of one or more of Mg, Y, Zn, and Zr.