Ferrite stainless steel for automobile exhaust system member superior in thermal fatigue strength

Abstract

A ferrite stainless steel for an automobile exhaust system member superior in thermal fatigue strength comprised of, by wt%, C: 0.020% or less, Si: 0.02 to 0.15%, Mn: 0.05 to 0.20%, P: 0.040% or less, S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.020% or less, Cr: 15 to 18%, Mo: 1.5 to 2.0%, Ti: 3x(C+N) to 0.25%, Nb: 0.4 to 0.8%, B: 0.0003 to 0.0050%, the C and N satisfying the relationship of C+N: 0.030% or less, and the Al, Si, and Mn satisfying the relationship of Alx(Si+Mn): 0.001 to 0.020, and a balance of Fe and unavoidable impurities. The 0.2% proof stress at 900 DEG C. before heat treatment in the atmosphere at 900 DEG C. for 300 hours is at least 15 MPa, and a difference of the 0.2% proof stress before and after the heat treatment is not more than 5 MPa.

Description

technical field [0001] The present invention relates to a ferritic stainless steel with excellent thermal fatigue properties used for automotive exhaust system components such as mufflers and exhaust manifolds. Background technique [0002] Due to increasing interest in environmental issues, improvement in the economy of fuel costs of automobiles and reduction in weight of vehicle bodies are strongly demanded, and purification of exhaust gas from automobiles is thus eagerly desired. Against such a background, stainless steel has been used for automotive exhaust system components. Among them, the exhaust manifold, which is one of the components that withstand the highest temperature, is repeatedly subjected to temperature rises up to about 1000°C and temperature drops to normal temperatures. Must have excellent heat resistance, especially excellent thermal fatigue properties. [0003] In recent years, the operating temperature of the exhaust manifold has increased, and the d...

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Problems solved by technology

[0007] However, it has been found that even with the invention disclosed in the aforementioned Japanese Patent No. 3021656, problems such as a decrease in high-temperature strength after high-temperature aging and a decrease in thermal fatigue properties cannot be sufficiently solved.

[0008] In addition, there is another problem. The high-temperature strength involved in the above-mentioned article CAMP-ISIJ Vol.16(2003) p544 by Hirasawa et al. is the initial high-temperature strength, and the invention described in this document does not propose any solution to the thermal fatigue characteristics

[0009] In addition, in the above-mentioned Japanese Patent No. 3242007, not only is there no description on the thermal fatigue characteristics, but also because the steel according to the invention described in this document has no addition of Al, contains only a small amount of Si, and contains almost no deoxidizing elements, Therefore, there is a problem that it is very difficult to deoxidize and precisely control the composition

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