JPS5970752A - Austenitic heat-resisting steel having excellent high temperature strength - Google Patents

Abstract

PURPOSE:To provide an austenitic heat resistance steel having excellent high temp. strength at a low cost by adding B, Nb, Ta, Ti and V, etc. together with Al to the austenitic heat resistance steel and using Cu in place of Ni. CONSTITUTION:A titled autenitic heat resistance steel contains <0.15% C, <1.0% Si, <2.0% Mn, 10.0-20.0% Ni+Cu, (8-18% Ni, <4% Cu), 13.0-20.0% Cr, 1.0- 2.6% Mo, and 0.02-0.5% Al, and contains further >=2 kinds among 0.002-0.005% B, 0.06-0.40% Nb + Ta, 0.06-0.15% Ti and 0.2-0.45% V. Al is added in order to prevent the formation of harmful nitride with B, Nb, Ti, Cr which are the elements to form carbide for improving strength. The base phase is reinforced by the mutual effect of Al and Mo. Further Cu is used in place of costly Ni. Such austenitic heat resistance steel is suitable as a material for the casing and valve body of an extra super critical pressure steam turbine.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an austenitic steel having excellent high-temperature strength, and particularly to an austenitic steel suitable for use in a casing or a valve body for an ultra-supercritical pressure steam turbine.

[Prior art]

In order to cope with the depletion of petroleum and the rise in oil prices, efforts are being made to improve efficiency by increasing the temperature and pressure of power plants. Steam power plants are currently operated at steam conditions of 538 C and 70 cages/g and valve body materials such as Or-Mo-
Ferritic heat-resistant steels such as V and 12 cr are used.

However, these ferritic heat-resistant steels have the drawback that in a temperature range of 550 C or higher, grain boundary warping is noticeable and the Schiffleop strength is extremely reduced, and it is difficult to use them under steam conditions of 600 G or higher.

C5US304 and 5US316 are austenitic heat-resistant steels currently used in the temperature range of 600C or higher.
There are steels, etc., but the 6501:10' creep rupture strength of these materials is about ``ff/+-''.The design requirement value i7-4~10.4 +(y f/g2?
On the other hand, there are examples in which carbide-forming elements such as Nb, Zr, and Ti are added singly to improve the wet strength of existing austenitic heat-resistant steels. However, these elements are more stable in nitrides than in carbides, so NbN, TiN
, ZrN, etc. are easily formed. Moreover, these nitrides have almost no solubility in the matrix and form large angular crystals within the grains.

Since it is distributed at grain boundaries, it is thought that it does not participate in precipitation hardening of the base metal. Therefore 1. l'Jb, Zr, Ti etc? The austenitic heat-resistant steel strengthened with additives satisfies the short-term creep rupture strength, but the long-term creep rupture strength is significantly reduced when I'J is absorbed from the atmosphere.

Furthermore, since Cr2N precipitates at grain boundaries near cracks, it has a negative effect on the fatigue life in which cracks propagate from the surface. For this reason, it is sometimes necessary to prevent the generation of nitrides in the steam turbine casing and valve body materials, which are subject to thermal fatigue due to startup and shutdown in addition to creep. Also, from the standpoint of design requirements for ultra-supercritical pressure steam turbine casings, 1
ncoly80 tJ, l 5-15N, (Jl
3B etc. N1 lfi hot steel satisfies the installation requirements, but it is considered to be difficult in terms of manufacturing technology to melt the casing material and large LC steel ingots.

[Purpose of the invention]

The purpose of this book is to develop an austenitic 1ill/j heat steel that is sufficient for casings or valve bodies for 600-650C ultra-supercritical pressure steam turbines, exhibits leap rupture, and has excellent high-temperature strength and is easy to build. I am happy to provide this service.

[Outline of the Invention] The present invention suppresses the harmful nitride precipitation of Ti, NN, NbN, TjN, and Cr2N4 by adding At, and suppresses the harmful nitride precipitation and the interaction between A4 and Mo. By strengthening the matrix and adding a stable carbide-forming element or Bk to improve creep rupture strength, and reducing the Ni content, the NiO content as an austenite-forming element d4
By using Cu, 2 as the A grade, the problems in manufacturing technology were solved.

The reasons for limiting the alloy composition in the present invention will be explained below. Note that the percentages described below are percentages by weight.

Carbon Combines with carbide-forming elements to form carbides, which precipitate and improve creep rupture strength. However, if added in a large amount, the toughness and M fluid properties will be significantly reduced.

Silicon is an essential deoxidizing component in manufacturing. However, if a large amount is added, it will adversely affect toughness, ductility, and weldability, so there is an upper limit. It shall be 1.0%.

Like silicon, manganese is an important deoxidizing component. but,
The lower the content, the better the oxidation resistance, so the upper limit is set at 2.0%.

Nickel is an important element that forms the r-stenite structure. If the amount is small, this effect is weak and the lower limit? 8. O%
41. On the other hand, Ni acts on Cr (!) and improves corrosion resistance, but it becomes expensive when used in large amounts, so its upper limit is 18.0.
%.

Copper is an austenite-forming element and has great M effect as a substitute for nickel. However, if added in a large amount, it will promote grain boundary embrittlement at pan temperature and increase high temperature cracking susceptibility, so the upper limit is set at 4.0%.

Chromium This is an important additive element for improving oxidation resistance, and since this effect is low in small amounts, the lower limit is set at 13.0%. However, if it is contained in a large amount, in addition to reducing weldability, it also has negative effects such as the formation of δ7 elite and promoting embrittlement during use at high temperatures, so the upper limit is 20.0%.
shall be.

Molybdenum dissolves solidly in the ground, strengthens the material, and combines with carbon to precipitate carbides, improving the rupture strength of the fiber. If it is less than 1.0%, this effect is low, but if it is used in a large amount, workability deteriorates, so the upper limit is set at 2.6%.

Aluminum Acts as a deoxidizer for molten metal and strengthens the parent phase by interacting with molybdenum. Further with nitrogen, i
, to force is high<NbN, i'iN, CrxN.

It is sometimes an important additive element for improving creep rupture strength in order to suppress the precipitation of harmful nitrides such as BN. In addition,
0.5% because aluminum is a ferrite-forming element
be the upper limit.

Each of the above elements is an essential component in the present invention υ, and each of the following elements is an optional component, and any 24! i1
or more is added.

Boron creep rupture strength, especially long-term creep strength r
It is an important element for improving. The effect is 0.00
It becomes noticeable at 2% or more. On the other hand, if added in a large amount, the strength will be adversely affected, so the upper limit is set to 'io, 005%.

Niobium, tantalum Forms stable carbonization and improves creep rupture strength. While this effect can be achieved with 0.06% or more, what happens if you add a large amount? f Totally reduces oxidation resistance. Therefore, the upper limit is set at 0.15%.

Similar to titanium and molybdenum, it is solid bathed in the base to strengthen the material and precipitate carbide to improve creep strength. This effect appears at 0.06% or more. However, niobium,
If tantalum is added, the oxidation resistance will be impaired, so the upper limit kO is set at 15%.

Vanadium is an effective element for improving corrosion resistance.
Its effect appears when it is 0.2% or more, but if it is added in a large amount, the austenite balance will be disrupted, so the upper limit is i0.45.
%.

[Embodiments of the Invention] FIG. 1 is a sectional view showing an example of a steam turbine suitable for use as the austenitic heat-resistant steel t-casing of the present invention.

In FIG. 1, a rotor 12 installed in a plurality of rotor blades 10 is installed in a plurality of static JKs so as to be located between the rotor blades 10.
The inner casing is provided with 1 flll shell through it. Then, the inner casing 16iJ,, i number of protrusions 18 are formed, and these protrusions 18 with a double rose pattern are fitted into the recesses of the outer casing 20 in which the inner casing is installed, and are fixed by the poles 14. . , and both ends of the rotor 12 at the outer casing 20 and the through hole portion 22? It is rotatably supported, and an outlet 24 is formed at the lower left in the figure, and an opening 26 is formed at the upper part.

The main steam flows through the main steam pipe 30 as shown by the arrow, and flows through the nozzle box 28? It flows into the inner casing 16 via Il-. Thereafter, when the main steam is rotated integrally with the dynamic rotor 12, the main steam enters the space between the inner casing 16 and the outer casing 2o, and flows out from the outlet 24.

Here, the main steam temperature is 650 tl' and the pressure is 350K.
If g f / crtl, the steam turbine is
Temperature 650c to 554.3 in inner casing 18
C% pressure 350 to 71/cJ ~ 199Ky f/cdl
, temperature s54.3c, pressure l in the outer casing
99k(yf/crA.

Maximum pressure of inner and outer casing M151 Kg f /cr
get d'.

Table 1 shows the required internal casing design strength for the above operating conditions.

Table 1 and Table 2 show the invention steel (Tl~T3) and comparative steel (81~S:
3) chemical composition basin is shown.

FIG. 1 shows the creep rupture strength of the invention steel and comparative steel at a test temperature of 650C. In FIG. 1, T1. Measured values of T2゜T3 steel,
A! 5! is the average strength of comparative steel? show. From Figure 1, A/
= together with B, Nb+i'a.

Any two metal elements Ti or V? Does the inventive steel with 6 addition have about 30% higher strength than comparative steel? I understand what is shown. The creep rupture strength of the steel of the present invention at 1000h and 10'h (calculated using the outer door) is nl 6 and 9f/
111111", about 10Kpf/mm"
It can be seen that the design requirements shown in the table are met.

In other words, B and N are added to the austenitic heat-resistant steel along with At.
b + Ta By adding two or more metal elements, Ti and V, it has a creep rupture strength approximately 30% higher than that of 5US316 steel, and also satisfies the design requirements for super-supercritical pressure turbine casings. do.

〔Effect of the invention〕

As described above, the present invention has high creep rupture strength, particularly long-term creep rupture strength, and is therefore effective as a material for a casing or a valve body for a supersupercritical pressure turbine.

[Brief explanation of the drawing]

FIG. 1 is a clear view showing an example of a steam turbine, and FIG. 2 is a diagram showing the 6500 clip rupture strength of the steel of the present invention and a comparative steel. 12... Rotor, 16... Internal casing, 20...
...External casing.

Claims (1)

[Claims] 1. In terms of weight ratio, C: 0.15% or less, Si: 1.0%
Below, Mo2 2.0% or less, Ni+Cu: 10.0~
20.0% (Ni: 8.0-18.0%, Cu:
4.0% or less), Cr: l 3.0 to 20.0%,
Contains Mo: 1.0-2.6%, At: 0.02-0.5%, further B: 0.002-0.005%, N
b10T a : 0.06-0.40, T i : 0
．． An austenitic heat-resistant steel having excellent high-temperature strength and containing two or more of V: 06 to 0.15% and V: 0.2 to 0.45%. 2. In claim 1, the main steam temperature is 600
~650tr, pressure 250-350 to 7f/cri
This austenitic heat-resistant steel is characterized by its high-temperature strength and is used in the casings and valve bodies of ultra-supercritical pressure steam turbines.