高窒素マルテンサイト鋼の熱処理硬さ，耐食性に及ぼすNおよびCの影響 Influence of Nitrogen and Carbon Contents on Quenched and Tempered Hardness and Corrosion Resistance of High Nitrogen Martensitic Stainless Steels

Nitrogen is one of the effective elements to enhance strength, corrosion resistance and austenite stability of stainless steel. Recently many studies have been conducted about nitrogen bearing austenitic, duplex and martensitic stainless steels. In this study, high nitrogen martensitic stainless steels containing 0.5C-0.2N, 0.3C-0.4N and 0.1-0.6N (; mass%) have been produced through the pressurized induction melting process. The hardness and corrosion resistance of the experimental steels have been investigated in various heat treatment conditions. The obtained results are as follows. (1) The maximum hardness of the steels after hardening with sub-zero treatment increases with carbon content, and the volume fraction of their retained austenite increases with nitrogen content. (2) In tempering process, all the steels show obvious secondary hardening at 723-773 K, and the maximum hardness increases with nitrogen content. Change in hardness of martensite after tempering at 723 K increases with [mass%N(in matrix)], ΔH723 K = 24.7 · [mass%N(in matrix)]-7.2 [HRC] where [mass%N(in matrix)] : nitrogen content in solid solution of as-quenched material. (3) The corrosion resistance of the steels after tempering is deteriorated with increasing tempering temperature. The steels containing more than 0.4 mass%N tempered under 723 K have corrosion resistance comparable with that of TYPE304 evaluated by pitting potential in 3.5 % NaCl aqueous solution.