JP3410139B2 - Al-containing oxidation-resistant austenitic stainless steel - Google Patents

Description

Detailed Description of the Invention

[0001]

[Field of Industrial Application] The present invention produces an Al-based oxide film on the surface in a high temperature oxidizing atmosphere of 800 ° C. or higher.
The present invention relates to an oxidation resistant austenitic stainless steel containing 6.0% Al.

[0002]

2. Description of the Related Art Component materials used at high temperatures, such as automobile exhaust manifolds, automobile exhaust gas reforming catalyst carriers,
Fe-Cr steel is used as a combustion member of heating equipment because it is required to have oxidation resistance in a use environment, mass productivity as an industrial product, and low cost.

For example, in ferritic stainless steel,
For automotive exhaust manifolds, 12% Cr-based stainless steel such as SUH409, 17% Cr-based ferritic stainless steel containing Nb and Cu, or 19% Cr-based ferritic stainless steel of the same system is used.

Further, the addition of Si reduces the growth rate of the Cr-based oxide film to improve the oxidation resistance, and the addition of 2.5% Si
It is also a well-known fact that 18% Cr-based ferritic stainless steel is often used in the combustion part of stoves.

On the other hand, Fe-Cr-Al alloy steel known as a heating wire, and more recently, 20% Cr-5 containing Ca or a rare earth element (REM) for use as an automobile exhaust gas reforming catalyst carrier.
% Al-based ferritic stainless steel foil and the like have an extremely excellent oxidation resistance by forming an Al ₂ O ₃ oxide film in a high temperature oxidizing atmosphere.

However, ferritic stainless steel has not been used in various plants, boilers, heat exchangers, heating furnaces, etc. due to its universal drawback of low high temperature strength.

On the other hand, austenitic stainless steel has high temperature strength and workability. For example, 25Cr-20Ni typified by SUS310S and 21Cr known as Incoloy 800.
−32.5Ni series, SUSXM15J1 with Si added, etc.
Each is used according to the purpose of use.

[0008] Fe-Cr-Ni alloy steel generally forms an oxide film mainly composed of Fe and Cr in a high temperature oxidizing atmosphere, but it has poor protection at 1000 ° C or higher. Further, in an environment in which heating and cooling are repeatedly performed, peeling occurs, and oxygen and nitrogen enter the base material, which accelerates deterioration of the material. There is an alloy to which Si or Al effective as an oxidation resistance improving element is added. Although the one with Si added has improved oxidation resistance compared to the unadded steel, the major oxide film is Fe, Cr-based oxides, and no dramatic improvement can be expected. Further, since SiO ₂ is generated at the Fe / Cr-based oxide / alloy interface, the peeling of the oxide film increases with repeated heating and cooling.

In view of such drawbacks of austenitic stainless steels, many are added with Al for the purpose of improving oxidation resistance. However, in austenitic stainless steels, diffusion of Al is slower than that in ferrite. Moreover, since Al in steel forms an intermetallic compound with Ni, the supply of Al to the surface becomes insufficient in a high temperature oxidizing atmosphere. There were few excellent properties.

As an example of improving the oxidation resistance by forming an Al ₂ O ₃ oxide film on the surface in a high temperature oxidizing atmosphere, Japanese Patent Laid-Open No. 52-
There are techniques disclosed in Japanese Patent No. 78612 and Japanese Patent Laid-Open No. 53-31517. By adding Al in excess of 4.0% to the steel, the oxidation resistance is improved by forming an Al ₂ O ₃ oxide film on the surface, which was not possible with conventional austenitic stainless steels. There is.

However, since the austenitic stainless steel fundamentally does not easily form an Al-based oxide film on the surface, it is not always possible to expect stable Al-based oxide film formation by adding 4.0% or more of Al.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to provide an austenitic stainless steel having oxidation resistance by forming an Al-based oxide film on the surface in a high temperature oxidizing atmosphere. More specifically, it is to develop a steel composition capable of forming an Al-based oxide film in Al-added austenitic stainless steel densely and stably.

[0013]

The inventors of the present invention aim to develop a material for an austenitic stainless steel having excellent oxidation resistance by forming a dense Al-based oxide film on the surface in a high temperature oxidizing atmosphere. The inventors of the present invention have studied the present invention, and have arrived at the present invention by obtaining the following findings.

By increasing the addition amount of Al in the steel, the oxide film formed changes from Fe and Cr-based to Al-based to improve the oxidation resistance. The effect of addition is large.

Therefore, if the addition amount of these elements is not optimized, it is impossible to form a dense and uniform Al-based oxide film with the conventionally-considered addition amount of Al. As a result of intensive studies on the influence of various elements in the steel on the formation of Al-based oxides, Si, which had been effective in improving the oxidation resistance of conventional Cr ₂ O ₃ -forming steels, became Al-based oxides. Has an adverse effect.

If the Si is not actively reduced, the formation of Cr-based and, in some cases, Fe-based oxides is permitted in a high temperature oxidizing atmosphere, and deterioration of the material occurs due to long-term use. By limiting Si to 0.15% or less, a healthy Al-based oxide can be easily formed, and oxidation resistance can be achieved even in the 2.5 to 4.0% Al-added region where it was previously thought that such an oxide film could not be formed. It was also found that an Al-based oxide having excellent properties can be densely and uniformly formed. Especially, it is the most important and characteristic in the present invention.

Therefore, the gist of the present invention is, by weight, C: 0.15% or less, Cr: 15 to 30%, Mn: 2.0% or less, Ni: 20 to 60%, Al: 2.5 to 6.0%. , Si: 0.15% or less,
The alloy composition consisting of the balance Fe and unavoidable impurities forms an oxide film mainly composed of Al on the surface in a high temperature oxidizing atmosphere at 800 ° C or higher.
It is an oxidation resistant austenitic stainless steel containing.

In the present invention, the above alloy composition is in a weight percentage.
In addition, Cr: 17 to 25%, and one or more rare earth elements or Ca may be contained in total of 1.0% or less. Also,
The alloy composition is, in wt%, one or more of Ti, Nb, and Zr, which is 4 times or more of C + N (%) and 2.0% or more.
It may be contained below. The alloy composition may be limited to wt% and may further be S (%) + O (%) ≤ 0.005 (%).

[0019]

Next, the reason why the present invention is limited to the above range will be described. C: C has the effect of forming Cr ₂₃ C ₆ type carbides at the time of application at high temperature or in the heat-affected zone of welding, and significantly reducing the workability and the effect of improving the oxidation resistance by Cr.
In addition, it is preferable that it is low because it causes scale peeling,
In the present invention, the upper limit is set to 0.15%.

Cr: Cr is a basic element necessary for obtaining oxidation resistance at high temperature together with Al. In the present invention,
The lower limit is 15% and the upper limit is 30%. Precise above 800 ° C
15% or more of Cr is required to form an Al-based oxide film. On the other hand, addition of more than 30% not only does not improve the oxidation resistance, but also adversely affects the formability and workability of the plate. It is preferably 17 to 25%.

Mn: Mn may be added to secure strength at high temperature. It is also effective for stabilizing the austenite phase. However, if added over 2.0%, the oxidation resistance is adversely affected, so 2.0% is made the upper limit.

Ni: Ni is an important element in providing the basic properties of austenitic steel. It is also necessary to increase the 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. Meanwhile, 60
If it exceeds%, it is difficult to put it into practical use in terms of cost, so the upper limit is set to 60%.

Al: Al is an important basic element in the steel of the present invention. 2.5% for stable formation of Al oxide film
The above is necessary. If it is less than 2.5%, Fe-Cr-Ni-based spinel type oxides are formed regardless of the Si content, and they are continuous.
Does not form an Al-based oxide film. However, if it is added in excess of 6.0%, not only the deformation resistance in the hot and the deterioration of the hot workability caused by the decrease of the grain boundary ductility due to the precipitation of NiAl intermetallic compound within the grains and grain boundaries, but also at room temperature Since the toughness of the steel becomes extremely remarkable, the upper limit is made 6.0%. It is preferably 3.0 to 5.5%.

Si: Si is an important element in the steel of the present invention. In order to form and maintain stably the Al-based oxide which is the steel of the present invention, it adversely affects. Therefore, the addition amount is limited to 0.15% or less.

Conventionally, Si is especially used as an element for improving the oxidation resistance of Cr.
₂ O ₃ It is added to the produced steel. Although the effect of improving the oxidation resistance by Si at that time is not clear, it acts as an indirect action to reduce defects in the main protective scale Cr ₂ O ₃ film and form pure and dense Cr ₂ O ₃ It is said that the growth rate of Cr ₂ O ₃ is reduced and the oxidation resistance is improved by forming SiO ₂ at the alloy interface with the Cr ₂ O ₃ oxide scale and suppressing the outward diffusion of metal ions. I have an idea.

On the other hand, in the presence of Al, which has a higher affinity for oxygen than Si, the formation of SiO ₂ inside the Al ₂ O ₃ scale is thermodynamically impossible. Therefore, there are few cases where the influence of Si was discussed in the alloy system that produces Al ₂ O ₃ scale. 20% Cr-5% used as an automobile exhaust system catalyst
From the results of examining the effect of Si addition on Al ferrite system, it was not found that the oxidation resistance was significantly affected.

However, the present inventors have found that Al-containing alloy
As a result of detailed and detailed experiments on the effect of Si on the stainless steel, it was found that the addition of Si in the steel deteriorates the oxidation resistance. An examination of the oxides produced by such materials with poor oxidation resistance revealed that Al ₂ O ₃ and Cr ₂ O ₃ were produced. Although there are many unclear points about the deterioration factor of the oxidation resistance, the results of the present inventors' study focusing on the base metal structure indicate that Si-added Ni-Al
It is conceivable that precipitation of the system-based compound increases and that a large amount of δ-ferrite phase appears. In the present invention, in addition to the above alloy elements, various alloy elements may be blended as necessary.

Rare earth elements (eg Y, Ce, La), Ca: These are oxidation resistance improving elements, and by fixing the sulfide in steel described later as a sulfide more stable than MnS, Improves workability. However, excessive addition adversely affects the oxidation resistance due to the formation of coarse oxide.
At least one of these elements is added in a total amount of 1.0% or less.

Ti, Nb, Zr: These elements are C in steel,
It is added to reduce the adverse effect of N and improve the workability and oxidation resistance. Ti, Nb, Zr are more C than Cr or Al,
Since it has a strong affinity with N, it acts as a stabilizing element. To fix C and N in steel, [C (%)
+ N (%)] more than 4 times more Ti, Nb or Zr
(Total amount) is required. On the other hand, excessive addition of Ti, Nb, and Zr brings about a decrease in toughness due to precipitation of intermetallic compounds, so the upper limit of the total amount of them is 2.0%.

S, O: S, O are limited in order to secure the cleanliness of the obtained steel. The S + O (%) value in steel is preferably low, but S + O (%) ≤ 0.008%, and more preferably S + O (%) ≤ 0.0050%.

More preferably, the upper limit of S is regulated to 0.002%, and if necessary, a rare earth element such as Ce, La, or Y that forms a more stable sulfide at a temperature higher than Mn, or Ca is added. Fix with. For the purpose of enhancing these effects, it is preferable that the O concentration in steel is low. This is because these additional elements tend to form oxides, are consumed as oxides before they function as S-fixing elements in steel, and the effective amount 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 composition as described above, but from the practical point of view in consideration of specifications and the like, the compositional adjustment is performed as follows. It is preferable to carry out.

N: N has the adverse effect of reducing the oxidation resistance at high temperature due to Cr and Al by forming a nitride by combining with Cr and Al in steel. The upper limit is 0.10%.

Mo: Mo may be added to secure strength at high temperature or to secure corrosion resistance. However, even if it exceeds 10.0%, not only the performance improvement is not seen, but also the deformation resistance at high temperature is increased.

P: P is not positively added. In principle, it is an impurity. Contains 0.03% or less. Cu: Cu in steel is allowed up to 1.5% as an impurity from the Ni source. Next, the present invention will be further described with reference to specific examples.

[0036]

[Examples] As shown in Table 1, various steel Nos. 1 to 30 having different Al and Si contents having alloy compositions were melted in a vacuum melting furnace and subjected to forging, hot rolling and cold rolling. The plate thickness was 2 mm. Various test materials were cut out from the thus-obtained steel plates and used as samples.

Of the steel types shown in Table 1, the invention steel Nos. 1 to 20 and the comparative steel Nos. 21 to 25, 28, and 29 were subjected to an oxidation test at 1000 ° C. in the atmosphere. The time-dependent change (mg / cm ² ) in the amount of oxidization of is shown in the graph in FIG. In addition, in the oxidation test, the oxidation resistance was evaluated based on the magnitude of the increase in the amount of oxidation after the oxidation including scale peeling.

In addition, for some steel types in the air atmosphere
After holding at 1000 ° C. for 200 hours, the change in the amount of oxidation increase per unit area was measured, and the oxide formed on the surface at this time was identified by thin film X-rays. Target is Cu, incident angle is α
= 0.3 °. Table 2 also shows the results of the scale cross-section observation and EPMA analysis.

FIG. 2 shows heating at 1100 ° C. for 30 minutes in the atmosphere.
It is a graph which shows the weight change of each sample steel which performed a cooling repetition oxidation test. From the time course of the increase in oxidation shown in FIG.
Each steel type having the proper chemical composition shown in the present invention is a comparative steel N.
Compared to o.21, 26, 28 and 29, it has better oxidation resistance and is a ferritic stainless steel with Al added, comparative steel No. 22 (Fe-20Cr
It has an oxidation resistance comparable to that of -5Al). Further, it can be seen that the material to which an appropriate amount of rare earth element, Ca or the like is added is more excellent in oxidation resistance.

From the results of thin film X-ray analysis, it is found that the steel of the present invention mainly contains α-Al ₂ O ₃ . On the other hand, among the comparative steels, No. 28, which has a higher Si content than the Al content,
29 is α-Al ₂ O ₃ and Cr ₂ O ₃ oxide scale, compared with comparative steel No. 21.
, 23 and 24 are FeCr ₂ O ₄ spinel type oxide and Cr ₂ O, respectively.
₃ , forming a multilayer scale of Fe ₂ O ₃ .

Further, from the results of the scale cross-section observation and EPMA analysis of the present invention steel and the comparative steel, the present invention steel is 2-3 μm.
The uniform oxide film of
In No. 8, a thick oxide film was formed in some places, and an Al ₂ O ₃ internal oxide layer was formed inside it. Further, it was confirmed that in Comparative Steel No. 25, a spinel type oxide film was formed on the surface, an Al ₂ O ₃ oxide layer was grown deep inside the base metal, and AlN was formed at the tip.

From the above, in the steel of the present invention, an Al-based main oxide film is uniformly formed on the surface in a high temperature oxidizing atmosphere, and then a stable and excellent protective film is obtained. Further, the repeated oxidation test results show that the film has excellent adhesion without causing scale peeling.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

As described above, according to the present invention, by optimizing the components of austenitic stainless steel having mechanical strength at high temperature, an oxide film mainly composed of Al is stably formed. It becomes possible to secure the oxidation resistance.

[Brief description of drawings]

FIG. 1 is a graph showing a change in oxidation weight gain in a continuous oxidation test of materials obtained in Examples.

FIG. 2 is a graph showing changes in weight of oxidized materials in a repeated oxidation test of materials obtained in Examples.

Claims (5)

(57) [Claims] 1. By weight%, C: 0.15% or less, Cr: 15 to 30%, Mn: 2.0% or less, Ni: 20
~ 60%, Al: 2.5-6.0%, Si: 0.15% or less, balance Fe and unavoidable impurities form an alloy composition to form an Al-based oxide film on the surface in a high temperature oxidizing atmosphere of 800 ° C or more. characterized by, 2.5 ~6.0% Al
Containing oxidation resistance austenitic stainless steel. 2. The alloy composition, in wt%, Cr: 17-25
%, And one or more rare earth elements or Ca,
The Al-containing oxidation resistance according to claim 1, wherein the total content is 1.0% or less .
Sex austenitic stainless steel. 3. The alloy composition, in% by weight, further comprising Ti,
The Al according to claim 1 or 2, which contains one or more of Nb and Zr in an amount of 4 times or more and 2.0% or less of C + N (%).
Containing oxidation resistance austenitic stainless steel. 4. The alloy composition, in wt%, further comprises S
The Al-containing oxidation-resistant austenitic stainless steel according to any one of claims 1 to 3, wherein (%) + O (%) ≤ 0.005 (%) . 5. The Al-containing oxidation-resistant austenitic stainless steel according to any one of claims 1 to 4, which further contains Mo: 10% or less by weight .