This book describes the details of the research and development of nickel-saving high nitrogen austenitic stainless steel and high nitrogen steel (HNS) with excellent corrosion resistance in the sea. This resistance was realized by using nitrogen (N), one of the ubiquitous elements, as an alloying element. By using N it is possible to save nickel (Ni), a national strategic substance in Japan, to be partially reduced or to be totally free. Not only basic information about low nickel HNS but also its promising application to bipolar plates of solid polymer fuel cells are provided. Reduction of Ni ions in the cell environment can be expected to extend the life of the fuel cell. Furthermore, there is discussion of the serendipitous application and impact of nickel-free HNS in the field of biomaterials as an anti-nickel allergy material, leading to its development for coronary stents. The book includes an explanation of how to develop extremely high corrosion-resistance steel by controlling its level of oxygen. For researchers and engineers involved in the development and evaluation of corrosion-resistant materials in particular, this book is extremely helpful for understanding the details of the development of those materials.