High Formability R429EX and Heat-Resistant R444EX Stainless Steels for Automotive Exhaust Manifold

摘要

To improve the formability and heat resistance of ferritic stainless steels for automotive exhaust manifolds, the effects of "the alloying elements C, Cr, Nb and mo on the high-temperature strength, toughnessz and high temperature structural stability were investigated, and the high-formability ferritic stainless-steel for general purpose R429EX (15 percent Cr-0.5 percent Nb steel) and the heat-resistance ferritic stainless steel for high exhaust gas temperature R444EX (19 percent Cr-0.6 percent Nb-2 percent Mo steel) were developed based zon the result of this investigation. The following knowledge was obtained from the development of, these steels; The addition of Mo and Nb is effective in improving high-temperature strength at temperatures 800 deg C and above. Although an increase in the Cr content raises strength, it does not affect high-temperature strength at temperatures 800 deg C and above. When high-temperature resistivity at 950 deg C is to be improved by the same degree by adding Mo to Nb, the deterioration in toughness is smaller with Mo addition than with Nb addition. Mo addition is not effective for suppressing grain coarsening after the heat treatment at 950 deg C for 200-h. A decrease in the C content or an increase in the Nb content is both effective in this respect. This seems to be due to the pinning effect of the grain boundaries by such fine precipitates as M_6C and Fe_2Nb. hnprrovment in high-temperature resistivity and ductility is both effective for improving the thermal fatigue characteristics. The high-formability, heat-resistant ferritic stainless steel for General purpose, R429EX, provides high-temperature strength equivalent to that of conventional type 430 JIL steel in the form of both sheet andf pipe and, at the same time, has high formability superior to that of type 430 JIL and close to that of type 409. Furthermore, the thermal fatigue characteristics of R429EX are superior to those of type 430JIL. After a long-time heat treatment at 950 deg C, the high-temperature characteristics of R444EX that was developed as a heat-resistant ferritic stainless steel for high exhaust gas temperatures are equivalent to those of type 430JIL after a long-time heat treatment at 900°C. Thus, R444EX snows outstanding high temperature strength and structural stability after heat treatment at elevated temperatures for a long time. Furthermore, the thermal fatigue characteristics of R444EX are about twice as good as those of type 430JIL. In addition, the formability and resistance to oxidation of R444EX are equivalent to those of type 430 JIL. R429EX expands the scope of application of stainless steel exhaust manifolds as a high-formability material for gneral purpose use that is suited to more complex shapes than those possible with the conventional steel material. R444EX is suited to higher temperatures and for thinner wall thickness of exhaust manifolds which are likely in the future. Both steels zare expected to be used in wide applications.