Development of high heat-resistant ferritic stainless steel with high formability, "RMH-1," for automotive exhaust manifolds by optimizing Mo composition design

To develop a high heat-resistant stainless steel with high formability for automotive exhaust manifolds, the influences of Mo and Si contents on the formability, oxidation resistance, and high temperature strength of 14%Cr ferritic stainless steel were investigated. Mo addition increased oxidation resistance and high temperature strength remarkably. Si addition increased significantly oxidation resistance but had a little effect on high temperature strength. Based on these findings, a high heat-resistant Mo-added ferritic stainless steel with high formability, RMH-1 (14.5%Cr-0.3%Si-0.5%Nb-1.6%Mo), has been developed. The newly developed steel possesses not only the advantage of the existing high heat-resistant type but also that of high formability type stainless steels. Namely, RMH-1 possesses high temperature strength and fatigue properties equal to those of a conventional SUS444 steel, R434LN2 (19%Cr-0.3%Si-0.35%Nb-1.8%Mo), which is considered to be a high heat-resistant steel for automotive exhaust applications, combined with formability equal to that of the conventional R429EX (14.5%Cr-0.9%Si-0.45%Nb), which is used as a high formability steel in the same applications.