KR800001346B1 - Heat-resisting austenite stainless steel - Google Patents

Abstract

An oxidn.-resisting austenitic stainless steel comprises<=0.15 Wt. % C, 1.5-4.0 Wt. % Si,<=2.0 Wt. % Mn, 17.0-30.0 Wt. % Ni, 24.0-32.0 Wt. % Cr, 0.5-2.5 Wt. % Al, 0.001-0.100 Wt. % of >=1 alkaline earth metal and the balance Fe. The alkaline earth metal is pref. Ca. The steel, for use in car exhaust systems, is resistant to oxidn. and sealing, have high temp. strength and is relatively inexpensive.

Description

Austenite heat resistant steel

The present invention relates to an austenitic heat-resistant steel having high strength even at high temperatures and easy processing at room temperature and having excellent economical efficiency and a wide range of uses.

The present invention relates to a special steel for use in applications that are heated continuously or intermittently in a high temperature atmosphere or in combustion gases. In particular, the present invention relates to an austenitic heat resistant steel having excellent oxidation resistance, nitriding resistance, and carburizing resistance at high temperature.

As the exhaust gas regulation of automobiles is specified, various stainless steels are attracting attention as heat-resistant steel for exhaust gas treatment systems. Among the exhaust gas treatment systems, an austenitic heat resistant steel is suitable as a material used in a combustor, a thermal reactor, etc., which generate a considerable high temperature in terms of strength at high temperatures and ease of processing at room temperature. Until now, this kind of material is a ferrite (Ferrite) -based Fe-Cr-Al alloy, the austenitic-based stainless steel such as SUS310S, and the high-grade NCF-2 and the like.

Fe-Cr-Al alloys have been studied for their use because of their excellent corrosion resistance and excellent thermal fatigue resistance, but they are disadvantageous for use because they are easy to deform due to low strength at high temperatures and poor welding and processing. have. On the other hand, SUS 310S is not good corrosion resistance and fatigue resistance compared to Fe-Cr-Al alloy, but other various properties are noteworthy.

However, austenitic heat-resistant steels such as SUS 310S, when intermittently heated in the atmosphere or in combustion gas, are reduced in thickness due to oxidation corrosion and further, nitriding proceeds remarkably. In addition, since a large amount of chromium nitride which exists in a metal by this nitriding precipitates and the amount of effective chromium decreases, corrosion resistance rapidly falls and it is difficult to use for a long time. For this reason, it was necessary to develop an austenitic heat resistant steel having excellent nitrification resistance and excellent corrosion resistance even after long use.

In view of this phenomenon, in the present invention, Si and Al are mixed and added, and a small amount of Ca or rare element is added to promote the uniform production of SiO ₂ and Al ₂ O ₃ during heating. By this action, an austenitic heat resistant steel having both nitriding resistance and corrosion resistance was developed, and the present invention was succeeded.

Prior to the present invention, austenitic heat-resistant steels having a high Si content include AISI 302B (18Cr-9Ni-2.5Si), AISI 314 (25Cr-20Ni-2Si), DIN 4828 (20Cr-12Ni-2Si), and the like. Has been used. Such metals show excellent oxidation resistance in the case of continuous heating at high temperature, and also excellent in nitriding resistance. However, when repeated heating and cooling, oxidative corrosion is generated and peeling is easy. Have. However, when a few percent Al is added to the austenitic heat-resistant steel and a very small amount of Ca and rare earth elements are added, the corrosion resistance is improved than when not added. However, when the Si content is low, the improvement of the nitriding resistance is hardly expected. For this reason, there is a drawback that corrosion resistance deteriorates rapidly during use.

However, as described above, the addition of Si and Al in combination with the addition of a very small amount of Ca and rare earth elements can significantly improve the corrosion resistance and increase the nitriding resistance even under repeated heating and cooling conditions.

The metal in the present invention is C: 0.15% or less, Si: 1.5-4.0%, Mn: 2.0% or less, Ni: 17.0-30.0%, Cr: 24% -32%, Al: 0.5-2.5%, and Ca: 0.001-0.100% and at least one rare earth element such as Y (yttrium), Ce (cerium), or La (lanthanum) are selected to contain 0.001-0.100%, and the remaining components are made of Fe and contain impurities unavoidable in manufacturing. It shall be. In addition, a small amount of one or more selected from Ti, Zr, Hf, Nb, Ta, etc. may be added to the component for the purpose of increasing the high temperature strength.

The reason for limiting the content of the alloying element in the metal of the present invention is as follows.

C: Austenite is an important element for obtaining high temperature strength, but if it is too large, hot working or cold working is difficult, so it is 0.15% or less.

Si: It is the most important element to improve high temperature oxidation resistance, carburizing resistance and nitriding resistance, and it requires 1.5% or more to obtain a composite effect with Al, but if it exceeds 4.0%, the composite effect does not increase any more. Cold workability is impaired.

As a Mn: austenite generating element, it can be used to save Ni, but since it is not good to contain a large amount because it impairs high temperature oxidation resistance, heat-resistant steel usually contains 2.0% or less.

It is one of the basic elements of Ni: austenitic heat-resistant steel, and has an effect of suppressing nitriding during heating. To maintain an austenite structure by containing an appropriate amount of Si and Al, more than 17.0% is required, but the upper limit is 30% due to economic efficiency.

Al: In the present invention, it is an important element for obtaining excellent corrosion resistance. A minimum of 0.5% is required to obtain the aluminum effect, but when it is contained in a large amount, the workability of the material is lowered and a large amount of Ni is required to adjust the balance of the components. For that reason it is good to contain 0.5-2.5%.

Cr: It is the most basic alloy element to maintain oxidation resistance at high temperature. If it is less than 24.0%, excellent properties cannot be obtained, and if it is more than 32.0% and an appropriate amount of Si and Al is added, a large amount of δ ferrite is produced and a large amount of Ni is required to maintain equilibrium, which is expensive. For this reason, it is good to contain 24% or more and 32% or less.

Ca: According to the present invention, when a small amount of Ca is added to an austenitic steel containing a large amount of Si and Al and subjected to a high temperature of oxidative conditions, SiO ₂ and Al ₂ O ₃ are mixed to form a uniform layered internal oxide. Production is promoted. As a result, the high temperature oxidation resistance is remarkably improved by suppressing the external diffusion of the cation of the metal. At the same time, the effect of suppressing nitriding can be obtained. In addition, 0.001% or more is required, but if it exceeds 0.100%, it is difficult to contain 0.001 to 0.100%.

Y. Ce. Rare earth elements such as La: Austenitic steels having a high Si content according to the present invention are hot-worked because they produce a few percent δ ferrite phase in the coalesced metal in order to reduce cracking susceptibility during hot welding. Even when a small amount of δ ferrite phase remains, cracks are likely to occur at high temperatures during processing. Trace amounts of rare earth elements are effective in preventing this kind of cracking. Like the effect of Ca, it is also effective for improving high temperature oxidation resistance. In particular, it is effective in improving corrosion resistance and has an effect of suppressing nitriding. In order to achieve such an effect, it is necessary to select at least one of rare earth metals (to be present as an equal component) and to contain 0.001-0.100%.

Ti, Zr, Hf, Nb, Ta: All are effective in increasing high temperature strength because they produce stable carbides and nitrides. These elements can suppress the formation of AlN and maintain an effective amount of Al in order to produce stable nitrides. Either 0.05-1.00% is good to contain.

Next, the characteristics of the inventive steel will be described in the Examples.

In Table 1, the inventive steel, the comparative steel with the prescribed range, and the commercially available steel are indicated by chemical components.

[Table 1]

In Table 2, oxidation-reduction value (mg / mg ² ) after 500 times of the comparative steel and invention steel shown in Table 1 and the process which heats 25 minutes at 1100 degreeC and 1200 degreeC and air-cools for 5 minutes, and after test, Nitrogen analysis values and nitrification rates during the test were recorded. According to these results, it can be seen that the oxidation loss is lower and the corrosion resistance is superior to that of commercial steels of Comparative Steels 2 and 3. Among them, the higher the Si and Al content as in Comparative Steels 2 and 3, the better the corrosion resistance. In addition, the addition of trace amounts of Ca and rare earth elements to steels with a relatively low Si content and a large Al content like Comparative Steel-4 shows excellent corrosion resistance. However, these comparative steels are the same as those of commercial steels, and since the nitriding proceeds remarkably during heating, a large amount of Cr ₂ N (or CrN) and AlN precipitate in the steel, thus reducing effective Cr and Al and significantly reducing corrosion resistance. . Invented steels having a high content of Si and Al and addition of a very small amount of Ca and rare earth elements are better than those of comparative and commercial steels and have improved corrosion resistance. In addition, since the progress of nitriding is slow, stable corrosion resistance can be maintained even when used for a long time.

[Table 2]

Table 3 shows the high temperature strengths of the present invention steels and commercial steels. As can be seen from the table, invention steels 4 and 5 added with Ti and Nb have higher temperature strengths than invention steels 1 and 2 and 3. have. The same effect can be expected for Zr, Ta, and Hf. In this way, it is preferable to add a small amount of Ti, Nb, Zr, Ta, Hf, etc. to increase the high temperature strength.

[Table 3]

As described above, the steel of the present invention has the characteristics of improving corrosion resistance, which is a drawback of the conventional austenitic steel having a high Si content, and hardly progressing nitriding during use. For this reason, like SUS310S, it is difficult to cause rapid deterioration of corrosion resistance by nitriding during use. In addition, as can be seen by comparison with the alloying elements described in the examples, the present invention steel can be expected to have excellent economic efficiency and wider range of use.

Claims (1)

An austenitic heat-resistant steel having the following composition. C: 0.15 or less, Ni: 17.0 to 30.0%, Si: 1.5 to 4.0%, Cr: 24.0% or more and 32.0 or less, Mn: 2% or less, Al: 0.5-2.5%, Ca: 0.001 to 0.100%, the sum of the selection of one or more of the rare earth metals is 0.001 to 0.100% (Also, one or more of Ti, Ta, Zr or Hf may be selected to contain 0.05 to 1.0% ) When the remainder is Fe but contains impurities which are unavoidable in manufacturing.