JPS6293352A - Austenitic heat resisting steel - Google Patents

Abstract

PURPOSE:To develop a heat resisting steel causing no deterioration in strength at high temp. even if temp. of steam to be used is elevated, by using an austenitic heat resisting steel containing specific amounts of Ti as a stock for blades and bolts of steam turbine. CONSTITUTION:The blades and bolts of steam turbine are manufactured out of austenitic heat resisting steel containing, by weight, <0.1% C, <0.5% Si, <2.0% Mn, 10-20% Cr, 30-50% Ni, 1.2-4.0% Mo, 2.0-4.0% Ti, 0.3-1.0% Al, 0.1-0.5% V, 0.001-0.01% B and <0.1% Zr. In this steel, Ti combines with Ni and Al to form an intermetallic compound [Ni3(TiAl)], which is finely distributed in a base alloy to improve its strength at high temp., so that, on application of this steel, no deterioration occurs in strength at high temp. of turbine blades and bolts even in a steam turbine driven by high-temp. steam of 600 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an austenitic heat-resistant steel, and more particularly to an austenitic heat-resistant steel suitable for turbine blades, bolts, etc. where the working fluid temperature is 600° C. or higher.

[Technical background of the invention and its problems]

Conventionally, in steam turbines where the steam temperature is 566°C or less, 120R base steel has been mainly used as the material for the trusses, blades, and bolts. However, from the standpoint of improving thermal efficiency, the steam conditions of steam turbines tend to be higher. In this case, in a high-temperature steam turbine where the steam temperature is 600° C. or higher, the conventional 12Or base steel does not have sufficient creep rupture strength, and a material with better high-temperature strength is required. 6
An example of a material that has excellent high-temperature strength at temperatures above 00°C is Fe-based heat-resistant alloy JISSUH660, which is strengthened by the γ' phase (Ni3(Al, Ti)).・In order to use it as a bolt, it is necessary to improve its high-temperature strength. In general, to improve the strength of Fe-based heat-resistant alloys strengthened by γ' phase) Io r T t *Al
Increasing the amount of steel is effective, but this is accompanied by a decrease in the instability of the austenite matrix and a decrease in ductility and toughness.

[Purpose of the invention]

The present invention has been made in view of the above points, and an object of the present invention is to provide an austenitic heat-resistant steel having excellent high-temperature strength and ductility and suitable for use in turbine blades and bolts where the working fluid temperature is 600°C or higher. shall be.

[A essential point of the invention]

In order to achieve the above object, the austenitic heat-resistant steel of the present invention has C0.1 or less and Si 0.1 in weight percent.
5 or less, M, n2. O or less, 0r10-20, Nl 3
More than 0 in 50 ma, Mo 1.2-4.0, Ti
2.0~4.0゜Afflo, 3~1.0, V 0
．． 1~0,5, B 0.001~0.01°Zr0
．． 1 or less, the balance is Fe-based austenitic heat-resistant steel consisting of Fe and incidental impurities.

The purpose of adding each component in the alloy and the reason for limiting the composition are as follows.

C is an essential element for increasing high-temperature strength, and as the amount of C increases, high-temperature strength improves, but on the other hand, if too much C reacts with Ti and forms Mo-type coarse carbides, the alloy deteriorates. Since the ductility of C is reduced, the amount of C is set to 0.1% or less.

Si acts as a deoxidizing agent, but when added in large amounts, N
Since it combines with Ti and Ti and precipitates as a G phase at grain boundaries, reducing the ductility of the alloy, it should be kept at 0.5% or less.

Mn acts as a deoxidizing agent, but if added in a large amount, the oxidation resistance deteriorates, so the amount added is set at 2.0- or less.

Or is an effective element for forming a healthy oxidation-resistant film and imparting oxidation and corrosion resistance to materials.
If it exceeds q6, O phase, which is a brittle phase, will be generated when used at high temperatures for a long time, so the content should be set at 10 to 20%.

Mo is an element that dissolves in solid solution in the austenite phase and is effective for solid solution strengthening, but if it is less than 1.2%, the effect cannot be obtained.
Also, if added in a large amount, the austenite matrix becomes unstable and the high temperature ductility decreases, so the range is 1.2 to 4.
Let it be 0chi.

Ti is the most important element for increasing the high-temperature strength j'IQ of the alloy of the present invention, and together with Ni and Al, it is an intermetallic compound γ.
' phase (Ni, (Ti, AA)) is produced and finely distributed in the base alloy, increasing the high temperature strength of the alloy of the present invention. However, if it is less than 2.0%, the effect will be small, and if it is added in a large amount, the η phase (Ni3Ti) will precipitate in layers from the grain boundaries, impairing ductility, so the content should be less than 4%.

Like Ti, Al is an important element for increasing the temperature and humidity strength of the alloy of the present invention, and combines with Ni to form the γ' phase (Ni
, (Ti, Affl)). It is also a useful element for imparting oxidation resistance, but 0.3%
If the content is less than 1.0%, the effect will be small (', if it is more than 1.0%, the processability will be deteriorated.

Ni is an element necessary for stabilizing austenite, and combines with Ti and AA to form the intermetallic compound γ' phase (
It is necessary to form Ni3(Ti,Al)) to improve high temperature strength. For this purpose, the above Or and M
It is preferable that the amount exceeds 30% for the range of 0.0%, and the larger the amount is, the more preferable it is, but since it becomes expensive, the upper limit is set at 50%.

■ is an element effective in increasing high-temperature notch toughness and forming carbides to improve high-temperature strength, but 0.1%
If it is less than that, the effect is not sufficient. Further, even if added in a large amount, it does not contribute to improving the strength, so it is set at 0.1 to 0.5%.

B segregates to the grain boundaries, is effective in strengthening the grain boundaries, and improves high-temperature ductility. For this purpose, 0.001% or more is required, but too much will impair hot workability, so the upper limit should be set to 0.001% or more.
．． 01%.

Zr is an effective element for strengthening grain boundaries and improving Loveture strength and high-temperature ductility, but on the other hand, if added in a large amount, it impairs hot workability, so it is limited to 0.1 inch or less.

The austenitic heat-resistant steel of the present invention can be obtained by the following procedure.

First, raw material metals are mixed and melted in a vacuum or in the atmosphere, and after deoxidation, a melted Fe-based heat-resistant alloy having substantially the above composition is obtained. This is then cast into a steel ingot,
It is preferable to apply vacuum arc remelting or electroslag remelting to this steel ingot in order to improve high-temperature ductility. The austenitic heat-resistant steel of the present invention can be obtained by forging or rolling the steel ingot produced in this manner and subjecting it to necessary heat treatment.

[Embodiments of the invention]

The present invention will be explained in detail by way of Examples below.

Three types of alloy samples having the compositions shown in Table 1 were each melted in a high frequency furnace, and remelted with engineered leftover slag using the samples as electrodes to obtain steel ingots. Roll this to 980
A test piece was taken from a material that had been subjected to solution treatment at 0C for 4 hours and aging treatment at 720C for 16 hours, and a tensile test and a creep-loveture test were conducted. The results are shown in Tables 2 and 3. Examples 1 and 2 of the present invention have the same or improved tensile strength and yield strength at room temperature and 650°C as compared to the comparative example, and the elongation and reduction of area are the same and no decrease in ductility is observed. On the other hand, it is understood that the creep rupture life in Table 3 is also significantly improved in Example 1.2 compared to the comparative example, while the elongation at break and the shrinkage are the same.

(Leaving space below) Table 2 Table 3 [Effects of the invention] As is clear from the test results of the above examples, the austenitic heat-resistant steel of the present invention exhibits superior high-temperature strength than conventional materials, and has sufficient strength. It has high temperature ductility. Therefore, the austenitic heat-resistant steel of the present invention has high reliability and is suitable as a material for blades and bolts of steam turbines that operate at high temperatures of 600° C. or higher.

Agent: Patent Attorney Noriyuki Chika Same as Hirofumi Mitsumata

Claims (1)

[Claims] Weight ratio: C0.1% or less, Si0.5% or less, Mn2
．． 0% or less, Cr10-20%, Ni over 30 and 50%
up to, Mo1.2-4.0%, Ti2.0-4.0%,
Al0.3-1.0%, V0.1-0.5%, B0.0
Austenitic heat-resistant steel consisting of 0.01 to 0.01%, Zr of 0.1% or less, and the remainder Fe and incidental impurities.