Ferritic Stainless Steel for Use as Conduit Members For Emission of Automotive Exhaust Gas

Abstract

A ferritic stainless steel useful as conduit members for emission of automotive exhaust gas consists of C up to 0.03 mass %, Si up to 1.0 mass %, Mn up to 1.5 mass %, Ni up to 0.6 mass %, 10-20 mass % of Cr, Nb up to 0.50 mass %, 0.8-2.0 mass % of Cu, Al up to 0.03 mass %, 0.03-0.20 mass % of V, N up to 0.03 mass % and the balance being Fe except inevitable impurities with a provision of Nb≧8(C+N). The steel may further contain 0.05-0.30 mass % of Ti and / or 0.0005-0.02 mass % of B. Mo as an inevitable impurity is controlled less than 0.10 mass %. The steel has excellent formability, low-temperature toughness and weldability as well as the same heat-resistance as Nb, Mo-alloyed steel.

Description

INDUSTRIAL FIELD[0001]The present invention relates to a ferritic stainless steel, which is excellent in heat-resistance, low-temperature toughness and weldability, useful as conduit members, e.g. exhaust manifolds, front pipes, center pipes and outer casings of catalytic converters in internal combustion engines of automobiles or the like, for emission of exhaust gas.BACKGROUND OF THE INVENTION[0002]Conduit members of automobiles for emission of exhaust gas are directly exposed to a high-temperature atmosphere containing exhaust gas during driving automobiles, and subjected to thermal stress caused by repetition of driving and stopping as well as vibration during driving. Mechanical stress is also applied to conduit members at a low temperature, when automobiles are started in winter in cold districts. Therefore, a material for use as the conduit members shall have durability in severe environments.[0003]Since conduit members are fabricated by welding or forming steel sheets or pip...

AI Technical Summary

Benefits of technology

[0007]The present invention aims at provision of a ferritic stainless steel useful as conduit members for emission of exhaust gas. An object of the present invention is to bestow a ferritic stainless steel, which does not contain expensive Mo, with heat-resistance similar to that of Nb, Mo-alloyed steel in addition to excellent formability, low-temperature toughness and weldability.

Problems solved by technology

Conduit members of automobiles for emission of exhaust gas are directly exposed to a high-temperature atmosphere containing exhaust gas during driving automobiles, and subjected to thermal stress caused by repetition of driving and stopping as well as vibration during driving.

However, poor formability and inferior low-temperature toughness are disadvantages of the Nb, Mo-alloyed steel.

Although a few reports are published on improvement of formability and low-temperature toughness, the improvement is still insufficient for the purpose.

Consumption of expensive Mo at a high ratio is also an disadvantage of the Nb, Mo-alloyed steel.

By the way, high-temperature strength (e.g. resistance to thermal fatigue failure) and high-temperature oxidation-resistance (evaluated as a critical temperature of abnormal oxidation) are not necessarily balanced at high level for some parts or members.

However, the Nb, Mo-alloyed steel is necessarily used for such a part or member with emphasis on heat-resistance regardless poor formability, inferior low-temperature toughness and expensiveness.

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