Nitrogen-Alloyed, Metastable Austenitic Stainless Steel for Automotive Structural Applications

The metastable austenitic stainless steels (MASSs) are the group of stainless steel with the austenitic microstructure that transforms readily into martensite during deformation. The strain-hardening rate increases markedly as martensite forms, and thereby these steels normally exhibit higher tensile strengths than the stable grades. The MASSs have been used in a variety of fields including structural applications such as railway passenger cars. Recently, growing interest arose on the use of these steels in automotive structural parts due to the need for increased fuel economy and crash safety of automobiles. The potential application of these steels includes space frames, chassis and structural parts, fuel tanks, and shock absorbing components. This expectation is based on the superiority of these steels in the strength to-weight ratio and a strain hardening capacity that cannot be readily overtaken by the current materials, i.e., carbon steels and aluminum alloys. Nitrogen alloying gives even higher strengths to the MASSs due to the well-known effect of nitrogen solution hardening. At the same time it makes the steels more economical than the normal Cr-Ni type MASSs since the nickel content of the steel can be considerably reduced through alloying nitrogen. In this article, the recent works on the automotive application of MASSs are briefly reviewed, and the mechanical properties of the new nitrogen-alloyed steel (POS 204M) for automotive applications are described. [Materials: 201, 204, 301L, 219, 310S, 204M, 6061, HSLA steel.]