JPS63183155A - High-strength austenitic heat-resisting alloy - Google Patents

Abstract

PURPOSE:To manufacture a high-strength austenitic heat-resisting alloy by incorporating specific percentages of C, Si, Mn, Cr, Ni, Mo, W, Nb, Ti, B, P, S, and N to Fe and also specifying the total content of Mo and W. CONSTITUTION:An alloy having a composition which consists of, by weight, 0.02-0.2% C, 0.3-1.5% Si, 0.3-1.5% Mn, 18-30% Cr, 20-50% Ni, 0.5-5.0% Mo, 1.0-5.0% W, 0.05-0.4% Nb, 0.01-0.2% Ti, 0.003-0.008% B, <=0.04% P, <=0.005% S, 0.02-0.3% N, and the balance Fe with inevitable impurities and in which Mo+W<=6.0% is satisfied is prepared. In this way, the austenitic heat-resisting alloy having superior high-temp. embrittlement characteristic, high-temp. corrosion characteristics, and weldability as well as excellent high- temp. strength and showing superior properties when applied to a boiler to be used under more service service environment can be obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention has extremely good high-temperature strength, as well as excellent high-temperature embrittlement properties, high-temperature corrosion properties, and weldability, and is suitable for use in increasingly harsh environments. This relates to an austenitic heat-resistant alloy that exhibits excellent performance when applied to certain boilers.

<Prior Art> Conventionally, austenitic stainless steels such as 5US347.5US316.5US310 have been widely used as materials used in high-temperature environments such as boilers.

However, with the depletion of energy resources and soaring prices in recent years, ultra-supercritical pressure boilers with steam conditions of high temperature and high pressure are being planned for thermal power plants in order to improve efficiency. The current austenitic stainless steel mentioned above is insufficient as a heat-resistant material that can withstand use under such harsh environments, and a material with even higher strength is required. As the material for this ultra-supercritical pressure boiler, as shown in Japanese Patent Application Laid-Open No. 59-173249,
Although alloys have been proposed that have higher high-temperature creep rupture strength than conventional heat-resistant stainless steel 5US347 and high-Ni stainless steel, and also take corrosion resistance and weldability into consideration, they are 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 has been developed to provide an inexpensive material that dramatically improves high-temperature strength compared to conventional austenitic heat-resistant alloys, and also has corrosion resistance and weldability. This was done to provide a heat-resistant alloy.

<Means for solving the problem> As a result of comprehensive judgment of various experimental facts, the inventors found that
We have succeeded in developing an inexpensive heat-resistant alloy that has dramatically improved creep rupture strength, as well as excellent corrosion resistance and weldability. That is, the present invention has a weight percentage of C0.02 to 0.02.
2%, Si 0.3-1.5%.

Mn0.3~1.5%, Cr18~30%, Ni20~
50%, Mo0.5-5.0%, W1.0-5.0%,
Nb0.05-0.4%, Ti0.01-0.2%, 8
0.003-0.008%, P0.04% or less, S
It is a high-strength austenitic heat-resistant alloy characterized by containing 0.005% or less, N0.02 to 0.3%, Mo+W≦6.0%, and the balance consisting of Fe and inevitable impurities.

The present invention will be explained in detail below. First, set the component range of C to 0
．． The reason for setting it at 0.02 to 0.2% will be explained below.

C is Cr, M0. It forms a carbide with W, Ti, Nb, and B, and its size, shape, and distribution have a great effect on creep rupture strength and elongation at break, so it is necessary to add the minimum amount necessary to form a carbide. lower limit to 0
．． 02%. On the other hand, in order to prevent hot cracking during welding and a decrease in ductility, it is necessary to lower Cl as much as possible, so the upper limit was set at 0.2%.

Si is an element that is not only effective as a deoxidizing agent but also improves oxidation resistance and high-temperature corrosion resistance, but if the amount of Ni is too large, it reduces creep rupture strength, toughness, and weldability. Therefore, the lower limit was set to 0.3% from the viewpoint of deoxidation, oxidation resistance, and high-temperature corrosion resistance, and the upper limit was set to 1.5% from the viewpoint of creep rupture strength, toughness, and weldability.

Mn is an element that has a deoxidizing effect and improves weldability and hot workability. In order to sufficiently deoxidize and obtain a healthy ingot, the lower limit was set at 0.3%. However, if the amount of Mn is too large, the oxidation resistance deteriorates, so the upper limit was set at 1.5%.

Cr is an essential element for oxidation resistance, steam oxidation resistance, and high-temperature corrosion resistance, and requires properties equal to or higher than those of conventional austenitic stainless steel. It was set as 18% of the amount. However, when the amount of Cr increases, the stability of austenite decreases, the high-temperature strength is weakened, and the formation of σ phase is promoted, resulting in a decrease in toughness, so the upper limit was set at 30%.

Ni is an essential element for increasing the stability of austenite and suppressing the formation of the σ phase. In order to maintain the stability of austenite with respect to the content of ferrite-forming elements such as Cr, Ni1l needs to be 20% or more. On the other hand, if the Ni amount exceeds 50%, there will be a disadvantage in terms of price, so the Ni amount was set to 20 to 50%.

M0. W is an element that has the effect of significantly increasing high-temperature strength through solid solution strengthening and carbide precipitation, but M.

If the amount is less than 0.5% and the amount of W is less than 1.0%, the effect cannot be obtained. Also M0. When W is added alone, Mo
When the amount and wB each exceed 5.0%, the effect is saturated. On the other hand, M0. When W is added in combination, Mo.

Due to the synergistic effect of W, the above effect is remarkable and the creep rupture strength is dramatically improved. However, the amount of Mo + W) is 6.
If it exceeds 0%, it promotes the formation of intermetallic compounds, tends to cause embrittlement over a long period of time, and is also disadvantageous in terms of processability and cost. Therefore, M0. W is added as a composite, the amount of Mo is 0.5 to 5.0%, the amount of W is 1.0 to 5.0%, and (
Mo+W) fit was set to 6.0% or less.

Nb and Ti form fine carbon and nitrides at the early stage of creep, and these form M! It promotes uniform and fine precipitation of 3Ch carbides and suppresses agglomeration and coarsening, significantly improving long-term creep rupture strength. However, if Nbfi is less than 0.05% and TiJ] is less than 0.01, the above effect cannot be obtained, so the lower limits of the amounts of Nb and Ti are set to 0.
．． 05%, 0. It was set as oi%. The above effect is more pronounced as the amount of Nb and Ti that can be dissolved in solid solution at the solution heat treatment temperature increases, but when Nh and Tt are added in excess of the solid solubility limit, undissolved carbon and nitrides remain. This causes the agglomeration and coarsening of M Z 3 Cb, resulting in a significant decrease in creep rupture strength.

Therefore, the upper limits of Nb amount and Ti amount are respectively 0.4%,
0.2%, solid solution (Nb + Ti) within that range
In order to increase the amount, Nb and Ti were added in combination. or,
In order to increase the amount of solid solution of Nb and Ti, the solution heat treatment temperature is desirably a high temperature of at least 1200°C or higher.

B is an element that has the effect of significantly improving creep rupture strength by increasing grain boundary strength, but...
If it is less than 0.008%, the effect will be small, and if it exceeds 0.008%,
Since weldability and hot workability deteriorate, the upper limit of the amount of B should be set to 0.
008%, and the lower limit was set to 0.003%.

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

S also segregates at grain boundaries, promoting grain boundary embrittlement during creep,
It also reduces hot workability, so the upper limit is set at 0.005.
%. N is an element that exhibits the effect of improving creep rupture strength through solid solution strengthening and nitride formation, but 0
．． If the amount of N is less than 0.02%, there is almost no effect; on the other hand, if the amount of N is 0.
．． Even if it exceeds 3%, the increase in long-term creep rupture strength is small and the toughness also deteriorates. Therefore, the range of N amount is set to 0.
02 to 0.3%.

Next, the present invention will be specifically explained using examples.

<Example> Table 1 shows the chemical composition of the test gold. These alloys are 1
After solution heat treatment at 250°C, a creep rupture test was conducted at 750°C, and the creep rupture strength at 700°C x 10'hr was determined by extrapolation using the Larson-Mi 1ler method. The test results obtained are also shown in Table 1. Among the alloys shown in Table 1, the A-G alloy is the invention alloy, and the H-R alloy is the comparative alloy. Among the comparative alloys, R alloy is a material equivalent to austenitic stainless steel 5OS347fl. Table 1 shows that the alloy of the present invention has a much higher creep rupture strength than SOS 347H, which is currently used as a material for boiler tubes.

Also, among the comparative alloys, H and I alloys have M0. The W-free alloy, the J alloy, and the Ni alloy are alloys containing only Mo and W, respectively, but compared to these alloys, the alloy of the present invention with a combined addition of Mo and W exhibits significantly higher creep rupture strength.

Among the comparative alloys, the L-Q alloy has 25Ni-20Cr as its basic components, and the L alloy has Nb, Ti, and B.

Those with composite addition of N, M alloys are Nb and Ti-free alloys, N alloys are Nb-free alloys, O alloys are Ti-free alloys,
P alloy is a B-free alloy, Q alloy is a low N alloy, but N
It can be seen that the creep rupture strength is significantly improved by the combined addition of Ti, B, and N.

<Effects of the Invention> As described above, the present invention enables the production of heat-resistant stainless steel MSUS3, which has been conventionally used as a material for high-temperature equipment such as boilers.
An austenitic heat-resistant alloy has been realized that has dramatically improved high-temperature creep rupture strength than 47 and high-Nt stainless steel, and also has corrosion resistance and weldability taken into consideration, and will greatly contribute to improving the performance and extending the life of ultra-supercritical pressure boilers. I can contribute.

Claims (1)

[Claims] C0.02-0.2% in weight percent, Si0.
3-1.5%, Mn0.3-1.5%, Cr18-30
%, Ni20-50%, Mo0.5-5.0%, W1.
0-5.0%, Nb0.05-0.4%, Ti0.01
~0.2%, B0.003~0.008%, P0.04
% or less, S0.005% or less, N0.02-0.3%, and Mo+W≦6.0%, the balance being Fe and inevitable impurities. .