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Improved formability and higher heat-resistance are increasingly demanded to materials used for hot end parts of automotive exhaust gas system in order to meet stricter emission regulations. A heat-resistant ferritic stainless steel and pipe with high formability was developed by refining the microstructure of hot rolled and annealed sheets. The newly developed steel showed a remarkable improvement in various forming properties such as limit drawing ratio, limit expansion ratio of pipe, and thickness reduction ratio of pipe after bending in comparison with conventional steels.

Automotive exhaust gas condensate causes severe corrosion inside mufflers. Corrosive condensate became a problem following the introduction of three-way catalytic converters in exhaust systems. The authors previously established a new laboratory test method that simulates condensate corrosion. This test method contributed to the development of new stainless steels for automotive mufflers. The addition of molybdenum (Mo) and copper (Cu) to ferritic stainless steels was found to be effective in improving condensate corrosion resistance, and the effect was arranged using the index [%chromium (Cr)+3×%Mo+1.5×%Cu]. A field test was carried out to investigate the corrosion behavior of mufflers under actual conditions. Aluminized steels showed severe damage by general corrosion. On the other hand, 19%Cr-Cu ferritic stainless steel exhibited excellent corrosion resistance. The corrosion behavior in the field test showed good agreement with the results of the laboratory test.