JPH0753898B2 - High strength austenitic heat resistant alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention has extremely good high temperature strength, and also has excellent high temperature embrittlement characteristics, high temperature corrosion characteristics, and weldability. It relates to an austenitic heat-resistant alloy that exhibits excellent performance when applied to a certain boiler.

<Prior Art> Conventionally, austenitic stainless steel such as SUS347, SUS316, and SUS310 has been widely used as a material used in a high temperature environment such as a boiler.

However, with the depletion of energy resources and soaring prices in recent years, an ultra-supercritical boiler in which steam conditions are increased in temperature and pressure is planned in a thermal power plant in order to improve efficiency. As the heat-resistant material that can withstand use in such a harsh environment, the above-mentioned conventional austenitic stainless steel is insufficient, and a material having high strength is required. As the material for this ultra-supercritical pressure boiler,
As disclosed in JP-A-59-173249, an alloy having higher high temperature creep rupture strength than conventional heat-resistant stainless steel SUS347 and high Ni stainless steel, and considering corrosion resistance and weldability has been proposed. However, it is still insufficient in terms of high temperature strength.

<Problems to be Solved by the Invention> In view of the above-mentioned circumstances, the present invention dramatically improves high-temperature strength as compared with conventional austenitic heat-resistant alloys and is inexpensive and has both corrosion resistance and weldability. It was made to provide a heat-resistant alloy.

<Means for Solving Problems> As a result of comprehensively judging various experimental facts, the inventors
We have succeeded in developing an inexpensive heat-resistant alloy with dramatically improved creep rupture strength and excellent corrosion resistance and weldability. That is, the present invention is C0.002-0.2%, Si in weight percent.
0.3 to 1.5%, Mn 0.3 to 1.5%, Cr18 to 30%, Ni20 to 50%, Mo0.
5 to 5.0%, W1.0 to 5.0%, Nb0.05 to 0.4%, Ti0.01 to 0.2%, B
0.003 to 0.008%, P0.04% or less, S0.005% or less, N0.02 to 0.3
%, Mo + W ≦ 6.0%, and the balance Fe and inevitable impurities, which is a high-strength austenitic heat-resistant alloy.

The present invention will be described in detail below. First, set the component range of C to 0.
The reason for setting 02 to 0.2% will be described.

C forms carbides with Cr, Mo, W, Ti, Nb and B, and its size, shape and distribution have a great influence on creep rupture strength and elongation at break. The lower limit was made 0.02% because it was necessary to add the minimum amount. On the other hand, it is necessary to reduce the C content as much as possible in order to prevent hot cracking and deterioration of ductility during welding, so the upper limit was set to 0.2%.

Si is an element that not only is effective as a deoxidizer but also improves oxidation resistance and high temperature corrosion resistance, but if the Si content is too large, creep rupture strength, toughness and weldability are reduced.
Therefore, in terms of deoxidation, oxidation resistance and high temperature corrosion resistance, the lower limit is set.
The upper limit was made 1.5% from the viewpoint of creep rupture strength, toughness and weldability.

Mn is an element that has a deoxidizing action and improves weldability and hot workability. The lower limit was made 0.3% in order to perform sufficient deoxidation and obtain a sound ingot. However, if the Mn amount is too large, the oxidation resistance is deteriorated, so the upper limit was made 1.5%.

Cr is an essential element for oxidation resistance, steam oxidation resistance, and high temperature corrosion resistance, and it requires characteristics equal to or higher than those of conventional austenitic stainless steels. 18% of the same amount. However, when the amount of Cr increases, the stability of austenite is lowered, the high temperature strength is weakened, the formation of the σ phase is promoted, and the toughness is lowered, so the upper limit was made 30%.

Ni is an essential element for enhancing the stability of austenite and suppressing the formation of σ phase. In order to stabilize the austenite with respect to the content of ferrite-forming elements such as Cr, the Ni content needs to be 20% or more. On the other hand, if the Ni content exceeds 50%, there is a disadvantage in terms of price, so the Ni content was set to 20 to 50%.

Mo and W are elements that have the effect of significantly strengthening the high temperature strength by strengthening the solid solution and precipitating carbides, but if the Mo content is 0.5% and the W content is less than 1.0%, that effect cannot be obtained. Further, when Mo and W are added individually, the above effects are saturated when the amounts of Mo and W each exceed 5.0%. On the other hand, when Mo and W are added together, the above effect is remarkable due to the synergistic effect of Mo and W, and the creep rupture strength is dramatically improved. However, the amount of [Mo + W] is
If it exceeds 6.0%, the formation of intermetallic compounds is promoted, embrittlement is likely to occur for a long time, and there is a disadvantage in terms of workability and cost. Therefore, Mo and W are added together, and the amount of Mo is 0.5-
5.0%, W content is 1.0-5.0%, and [Mo + W] content is 6.0%
Below.

Nb, Ti is the initial creep, forms fine carbonitride material, they,促Gashi uniform-fine precipitation of M ₂₃ C ₆ carbides, for suppressing the aggregation and coarsening, long-term creep rupture strength To improve significantly. However, if the Nb content is less than 0.05% and the Ti content is less than 0.01, the above effect cannot be obtained.
The lower limits of i amount were set to 0.05% and 0.01%, respectively. The above effect is more remarkable as the amount of Nb and Ti that can be solid-solved at the solution heat treatment temperature is larger, but when added in excess of the solid solution limit of Nb and Ti,
Undissolved carbon / nitride remains, causing coagulation and coarsening of M ₂₃ C ₆ and significantly lowering the creep rupture strength. Therefore, the upper limits of the amount of Nb and Ti were set to 0.4% and 0.2%, respectively, and Nb and Ti were added in combination in order to increase the amount of solid solution [Nb + Ti] within the ranges. Moreover, in order to increase the amount of solid solution of Nb and Ti, the solution heat treatment temperature is preferably high at least 1200 ° C or higher.

B is an element that has the effect of significantly improving creep rupture strength as a result of increasing the grain boundary strength, but if it is less than 0.003%, the effect is small, and if it exceeds 0.008%, the weldability and hot workability deteriorate. , B amount upper limit is 0.008%, lower limit is 0.0
It was set to 03%.

When P is added in a large amount, it promotes precipitation during creep and promotes embrittlement during creep, so the upper limit was made 0.04%.

S also segregates at the grain boundaries, promoting grain boundary embrittlement during creep,
Further, the hot workability is also deteriorated, so the upper limit was made 0.005%. N is an element that has the effect of improving creep rupture strength by solid solution strengthening and formation of nitrides, but 0.02%
If it is less than 0.1%, there is almost no effect. On the other hand, if the amount of N exceeds 0.3%, the creep rupture strength does not increase for a long time and the toughness also deteriorates. Therefore, the range of the amount of N is set to 0.02 to 0.3%.

Next, the present invention will be specifically described with reference to examples.

<Examples> Table 1 shows the chemical composition of the match money. 12 these alloys
After solution heat treatment at 50 ° C, a creep rupture test was performed at 750 ° C, and the creep rupture strength at 700 ° C x 10 ⁵ hr was extrapolated by the Larson-Miller method. The test results obtained are also shown in Table 1. Among the alloys shown in Table 1, the A to G alloys are the alloys of the present invention, and the H to R alloys are the comparative alloys.
Among the comparative alloys, R alloy is austenitic stainless steel SU
Equivalent to S347H. It can be seen from Table 1 that the alloy of the present invention has much higher creep rupture strength than SUS347H which is currently used as a boiler tube material.

Among the comparative alloys, the H and I alloys are Mo and W-free alloys, and the J and K alloys are Mo and W single-added alloys, respectively. Compared to these alloys, Mo and W were added in combination. The alloys of the present invention exhibit remarkably high creep rupture strength.

Among the comparative alloys, the L to Q alloys have 25Ni-20Cr as a basic component, and the L alloy has Nb, Ti, B and N added in combination,
M alloy is Nb and Ti-free alloy, N alloy is Nb-free alloy, O
Although the alloy is a Ti-free alloy, the P alloy is a B-free alloy, and the Q alloy is a low-N alloy, it can be seen that the creep rupture strength is remarkably improved by the combined addition of Nb, Ti, B, and N.

<Effects of the Invention> As described above, according to the present invention, the high temperature creep rupture strength is remarkably improved as compared with the heat resistant stainless steel SUS347 and the high Ni stainless steel that have been conventionally used as materials for high temperature equipment such as boilers, and An austenitic heat-resistant alloy with sufficient consideration given to corrosion resistance and weldability is realized, which can greatly contribute to improving the performance and extending the life of the ultra-supercritical boiler.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Araki 3434 Shimada, Hikari-shi, Yamaguchi Pref., Nippon Steel Co., Ltd. Inside the Hikari Works (72) Inventor, Hidehiro Hota 5-10-1, Fuchinobe, Sagamihara-shi, Kanagawa (2) Inventor Toshio Fujita 1-14-4 Mukogaoka, Bunkyo-ku, Tokyo

Claims (1)

[Claims] 1. In weight percent, C0.02 to 0.2%, Si0.3 to 1.5%, Mn0.3 to 1.5%, Cr18 to 30%, Ni20 to 50%, Mo
0.5-5.0%, W1.0-5.0%, Nb0.05-0.4%, Ti0.01-0.2
%, B0.003 to 0.008%, P0.04% or less, S0.005% or less, N0.02
~ 0.3%, Mo + W ≦ 6.0%, balance Fe
And a high-strength austenitic heat-resistant alloy, characterized by comprising unavoidable impurities.