Ferritic stainless steel and ferritic stainless steel for automobile exhaust gas passage member

Abstract

The present invention relates to a ferritic stainless steel containing 0.03% by mass or less of C; from 0.1 to 0.8% by mass of Si; 1.0% by mass or less of Mn; 0.04% by mass or less of P; 0.01% by mass or less of S; 0.5% by mass or less of Ni; from 12.0 to 15.0% by mass of Cr; 0.03% by mass or less of N; from 0.1 to 0.5% by mass of Nb; from 0.8 to 1.5% by mass of Cu; and 0.1% by mass or less of Al, the balance being Fe and unavoidable impurities, the ferritic stainless steel having a γ max of 55 or less, as represented by the following equation (1):γ max=420C−11.5Si+7Mn+23Ni−11.5Cr+470N+9Cu−52Al+189  (1),in which C, Si, Mn, Ni, Cr, N, Cu and Al mean % by mass of the corresponding elements.

Description

TECHNICAL FIELD[0001]The present invention relates to a ferritic stainless steel and a ferritic stainless steel for automobile exhaust gas passage members.BACKGROUND ART[0002]Ferritic stainless steels are used in heat-resistant applications where thermal distortion is problematic, because they have a lower thermal expansion coefficient, better thermal fatigue characteristics and better high-temperature oxidation characteristics as compared with austenitic stainless steels. Typical applications of the ferritic stainless steels include automobile exhaust gas passage members such as exhaust manifolds, front pipes, outer cylinders of catalyst supports, center pipes, mufflers and tail pipes.[0003]Recent automobile engines tend to increase a temperature of an exhaust gas in order to improve an exhaust gas purification efficiency and an output, and particularly high heat resistance (high-temperature strength, high-temperature oxidation resistance) is required for members close to the engin...

AI Technical Summary

Benefits of technology

The present invention provides a ferritic stainless steel with improved processability, heat resistance, and surface quality, which is suitable for use in automobile exhaust gas passage members.

Problems solved by technology

Also, recently, a shape of the exhaust gas passage member tends to be complicated.

The complicated shape leads to concentration of thermal strain at one point due to the start and stop of the engine so that thermal fatigue failure tends to take place, as well as leads to a local increase in a temperature of the material so that abnormal oxidation also tends to take place.

Therefore, the heat resistance cannot be sacrificed in order to improve formability.

However, in view of the heat resistance level, it is not preferable to apply it to applications where the temperature of the material is more than 800° C. On the other hand, SUS430J1L has good heat resistance, which can be used at 900° C. However, since it is hard, any application may be difficult in terms of processability.

However, the maintenance in a Cu precipitation temperature range for a long period of time leads to coarse precipitates of Cu due to agglomeration of the precipitates, resulting in a decreased effect of improving the high-temperature strength.

However, since the ferritic stainless steel has the increased γ max, the martensitic phase may be formed when heated at an elevated temperature as in welding, whereby the thermal fatigue characteristics may be degraded.