JP3006337B2 - Heat-resistant austenitic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a heat-resistant austenitic stainless steel having excellent oxidation resistance and high-temperature strength in a high-temperature oxidizing atmosphere of 800 ° C. or higher.

[0002]

2. Description of the Related Art Austenitic stainless steel has high temperature strength and workability, for example, SUS310S as a representative.
25Cr-20Ni, 21Cr-3 known as Incoloy 800
There are 2.5Ni type and SUSXM15J1 to which Si is further added, and each is selected and used according to the purpose of use.

[0003] However, today's demands are becoming more and more severe depending on the application, and there is a limit in the oxidation resistance to meet such conventional materials. In order to improve the oxidation resistance of such austenitic stainless steel, Al
By adding, an oxide film mainly composed of Al, which could not be formed conventionally, is formed on the surface, thereby improving the oxidation resistance. .

However, when such a material is used in a high temperature environment of 800 ° C. or more, such as a superheater tube, a heat treatment furnace, and a heat exchanger, high strength is required at the same time. In recent years, due to the environmental problems including the depletion of petroleum resources and air pollution in the 21st century, fuel cells that can use coal-reformed gas as a next-generation power supply source and have high energy conversion efficiency are beginning to attract attention. For this reason, research and development of the entire power generation device, including auxiliary machines surrounding the main body, has been conducted in order to improve the overall power generation efficiency and to make the device compact, as well as the main body that generates electromotive force.

[0005] In particular, some heat exchangers used in solid oxide fuel cells have an environment near 1000 ° C, and require a material having oxidation resistance and high-temperature strength for a long time. In addition, ductility and toughness when used for a long time are also important.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide an austenitic stainless steel which has stable oxidation resistance over a long period of time in a high-temperature oxidizing atmosphere of, for example, 800 ° C. or higher and has high strength at high temperatures. is there.

More specifically, an object of the present invention is to provide a material suitable for use in a high-temperature environment of 800 ° C. or more, such as a superheater tube, a heat treatment furnace, and a heat exchanger. It is to provide an austenitic stainless steel having high strength.

[0008]

SUMMARY OF THE INVENTION An austenitic stainless steel, which is required to have such excellent high-temperature characteristics,
A material that has excellent oxidation resistance over a long period of time has been developed by optimizing the component balance between Al and Si to more stably form an oxide film mainly composed of Al in a high-temperature oxidizing atmosphere. Furthermore, through subsequent development research,
It is possible to form an Al-based oxide film even in the 2.5 to 4.0% Al region, and to obtain a good oxidation-resistant material that ensures workability.
Al and Si optimized austenitic stainless steels have also been developed.

However, it has been found that when such austenitic stainless steel having excellent oxidation resistance is used in the above-described high-temperature environment, high-temperature strength and high ductility during long-term use become problems.

The present inventors have intensively studied to develop a material of austenitic stainless steel having excellent oxidation resistance and high-temperature strength in the high-temperature oxidizing atmosphere as described above. The present invention has been achieved by obtaining such insights. In other words, (1) Austenitic stainless steel containing 2.5% or more Al by oxidation of Al and Si has excellent oxidation resistance in a high-temperature oxidizing atmosphere at 800 ° C or higher. Strength,
There is a problem with high ductility when used for a long time. (3) As a result of high temperature property tests using various component changing materials, Si and N in steel have an effective interaction for high temperature strength, and the limitation of Si increases high ductility. (4) The addition of B improves the high-temperature strength by strengthening the grain boundaries, and (5) the composite addition of these results in a higher strength material.

Therefore, the gist of the present invention is that, by weight%, C: 0.15% or less, Si: 0.15% or less, Cr: 15 to 30%
%, Mn: 5.0% or less, Ni: 20-60%, Al: 2.5-6.0%,
N: 0.10 to 0.35%, with the balance being Fe and unavoidable impurities, heat-resistant austenitic stainless steel. According to a preferred embodiment, the steel further comprises:
By adding B: 0.0005 to 0.020%, the high-temperature strength can be further improved.

[0012]

Next, the reason why the present invention is limited to the above range will be described. C: C has the effect of forming a Cr ₂₃ C ₆ type carbide at the time of application at a high temperature or in the weld heat affected zone, thereby significantly reducing the effect of improving workability and oxidation resistance by Cr.
In addition, a lower one is preferable to cause scale peeling,
In the present invention, the upper limit is set to 0.15%. However, when emphasis is placed on the strength at high temperatures, the content may be increased to near the upper limit.

Si: Si is the most important element in the steel of the present invention. If the content exceeds 0.15%, oxides mainly composed of Al cannot be stably generated in a high-temperature oxidizing atmosphere.

Cr: Cr is a basic element necessary for obtaining oxidation resistance at high temperatures together with Al. In the present invention,
The lower limit is 15% and the upper limit is 30%. Over 800 ℃
15% or more Cr is necessary to form an Al-based oxide film. On the other hand, addition of more than 30% not only does not show improvement in oxidation resistance, but also adversely affects plate formability and workability.

Mn: Mn may be added to ensure strength at high temperatures. It is also effective for stabilizing the austenite phase. However, if added over 5.0%, the oxidation resistance is adversely affected, so the upper limit is 5.0%.

Ni: Ni is an important element for providing the basic properties of austenitic steel. It is also necessary to increase high-temperature strength and high-temperature creep strength near 1000 ° C. If it is less than 20%, the austenite phase becomes unstable, and a single Al-based protective film cannot be formed. On the other hand, 60
%, It is difficult to put into practical use in terms of cost. The upper limit is 60%.

Al: Al is an important basic element in the steel of the present invention. If the Al-based oxygen film is to be formed stably, it must be at least 2.5%. If it is less than 2.5%, a Fe-Cr-Ni-based spinel oxide is generated regardless of the amount of Si, and continuous Al
Does not form a system oxide film. However, if it is added in excess of 6.0%, not only the deformation resistance during hot working and the deterioration in hot workability caused by the decrease in grain boundary ductility due to precipitation of NiAl intermetallic compounds within grains and at grain boundaries increases, but also at room temperature. Since the decrease in toughness becomes extremely noticeable, the upper limit is made 6.0%.

N: N is one of the most important elements in the present invention. It is an austenite-forming element and forms a solid solution in austenitic ground to provide a strengthening effect. 0.10%
The effect is exhibited by the above addition. But 0.35%
If added in excess of, the workability is degraded.

S, O: S forms a compound with Mn in steel and becomes a starting point of abnormal oxidation. Further, grain boundary segregation at the time of solidification lowers the ductility of the grain boundaries and causes cracks during hot working. Therefore, the upper limit is regulated to 0.0030%, preferably 10 ppm, and, if necessary, immobilization is performed by adding a rare earth element such as Ce, La, Y or the like which forms a more stable sulfide at a temperature higher than Mn or Ca. .

In order to enhance the effects of these additional elements to ensure hot workability, the oxygen concentration in the steel is preferably low. This is because these additional elements tend to form oxides, are consumed as oxides before functioning as S-fixing elements in steel, and reduce the effective amount. S + in steel
The lower the O (%) value, the better, but S + O (%) ≦ 0.0080
%, More preferably S + O (%) ≦ 0.0050%. In the present invention, B may be added if necessary.

B: B improves the high-temperature strength by strengthening the grain boundaries, but if less than 0.0005%, the effect cannot be obtained.
%, The forgeability decreases. The composition of the austenitic stainless steel to which the present invention is applied is not particularly limited as long as it has the above-described composition, but from a practical viewpoint in consideration of standards and the like, component adjustment may be performed as follows. preferable.

Ti, Nb, Zr: Cr or Al for reducing the adverse effects of C and N in steel and improving workability and oxidation resistance.
Alternatively, a stabilizing element such as Ti, Nb, or Zr having a higher affinity for C and N may be added. However, excessive addition of Ti, Nb, and Zr causes a decrease in toughness due to precipitation of an intermetallic compound. Therefore, the upper limit of Ti + Nb + Zr (%) is set to 2.0%.

Rare earth elements (Examples: Y, Ce, La), Ca: These are elements for improving oxidation resistance, and further, by fixing sulfide in steel described later as sulfide more stable than MnS, Improve workability. However, excessive addition adversely affects oxidation resistance due to the formation of coarse oxides.
Furthermore, since the hot workability deteriorates rapidly, the total amount
Add in a range of 0.50% or less.

Mo: Mo may be added for securing strength at high temperatures or securing corrosion resistance. However, even if it exceeds 10.0%, further improvement in performance is not seen, and deformation resistance at high temperature is increased. P: P is not actively added. In principle, it is an impurity. It contains 0.03% or less.

Cu: Cu in steel is an impurity from a Ni source.
Up to 1.5% is acceptable. Next, the present invention will be further described with reference to specific examples.

[0026]

【Example】

(Example 1) A creep rupture test at 1000 ° C was performed using test materials in which each of the main components was changed with the steel composition of No. 1 in Table 1 as a basic component. FIG. 1 shows the relationship between the creep rupture time and rupture elongation and the Si content, and FIG. 2 shows the relationship between the creep rupture time and the N content. From these results, it is possible to improve the rupture life and rupture ductility by reducing the content of Si to 0.5% or less. Further, it was found that the test material having such Si content had excellent creep rupture characteristics when the N content was 0.10 to 0.35%.

Example 2 Next, steels having various chemical components shown in Table 1 were tested. Steel Nos. 1 to 9 are steels of the present invention and N
o.10 to 23 are comparative steels. Of these, steel Nos. 1 to 9, 1
Samples 3 and 23 were melted in a vacuum melting furnace, forged and hot-rolled to a thickness of 5 mm, and then subjected to a solution treatment at 1200 ° C.

These test materials were also used at 850 ° C. and 1000 ° C.
A creep rupture test at ℃ and a high temperature tensile test were performed.
The results are shown in Table 2. FIG. 3 shows the change over time (mg / cm ² ) of the increase in oxidation per unit area of the steels of the present invention and Comparative Steels Nos. 10, 15, and 21 among the steel types shown in Table 1 at 1000 ° C. in the atmosphere.

The oxidized test material was 2 t × 20 × 25 mm, and the test was carried out using No. 600 emery paper after polishing the end face, surface polishing and degreasing. In the oxidation test, the oxidation resistance was evaluated based on the amount of increase in oxidation after oxidation including scale peeling.

From the results shown in Table 2, it can be seen that each steel type having the proper components according to the present invention is superior in the high-temperature strength characteristics as compared with the comparative steel. Further, the results of the oxidation resistance test show that the steel of the present invention has excellent oxidation resistance in a high-temperature oxidation atmosphere, and it has been confirmed that the steel has both oxidation resistance and high strength at high temperatures.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

As described above, according to the present invention, since it has excellent oxidation resistance for forming an oxide film mainly composed of Al and has excellent strength characteristics at a high temperature, it has a high temperature of 800 ° C. It is extremely promising as a material for a heat exchanger used in an oxidizing atmosphere.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between creep rupture time and ductility at 1000 ° C. and the Si content in steel in the same system as the steel of the present invention.

FIG. 2 is a graph showing the relationship between creep rupture time at 1000 ° C. and N content in steel in the same system as the steel of the present invention.

FIG. 3 is a graph showing changes in the amount of increase in oxidation in a continuous oxidation test of the material obtained in Example 2.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C22C 19/05

Claims (2)

(57) [Claims] C. 0.15% or less, Si: 0.15% or less, Cr: 15 to 30%, Mn:
5.0% or less, Ni: 20 to 60%, Al: 2.5 to 6.0%, N: 0.10 to 0.35% or less, S: 0.0030% or less and S + O: 0.0080% or less, balance Fe and inevitable impurities Heat resistant austenitic stainless steel. 2. B: 0.0005 to 0.020 by weight%
The heat-resistant austenitic stainless steel according to claim 1 containing at most 10% by weight.