Abstract: A process for manufacturing a high nitrogen stainless steel pipe material includes keeping an outside surface and/or an inside surface of an austenite stainless steel pipe material in contact with a substance that becomes a nitrogen (N) source, heating the steel pipe together with the nitrogen source substance at a temperature of 800° C. to 1100° C. in a range of temperatures not higher than the critical temperature for crystal grain enlargement of the steel pipe material to cause nitrogen to be absorbed into the surface of the pipe and diffused into the steel solid phase, and applying to the heat-treated pipe material annealing treatment in the range of temperatures in vacuum, inert gas including argon gas or an atmosphere of a gas with a reducing substance including H2 gas added thereto, to result in a decrease of nitrogen concentration gradient.

Abstract: Nitrogen (N) absorption and diffusion treatments are performed for the inner and/or outer surfaces of austenite stainless steel pipe materials in N gas atmosphere at temperatures near 1,100° C. to obtain nitrided stainless steel pipe materials having 0.25˜1.7% (mass) of solid solution nitrogen (N) including a gradient structure formed within the pipe wall in which the concentration of solid solution N continuously decreases gradually from the surface. The solid solution N present in the gradient structure promotes short range ordering (SRO) of substitutional alloying elements leading to homogenization of distribution of alloying elements in the austenite phase, generating an extremely high proof strength (yield strength) about 3 times as high as that of conventional austenite stainless steel pipe materials and enhancing characteristic of anti-hydrogen gas embrittlement (anti-HGE) so as to be suitable for use in a high pressure hydrogen tank utilized in hydrogen cell vehicle (FCV) and a liquid hydrogen tank.

Abstract: A process for manufacturing a high nitrogen stainless steel pipe material includes keeping an outside surface and/or an inside surface of an austenite stainless steel pipe material in contact with a substance that becomes a nitrogen (N) source, heating the steel pipe together with the nitrogen source substance at a temperature of 800° C. to 1100° C. in a range of temperatures not higher than the critical temperature for crystal grain enlargement of the steel pipe material to cause nitrogen to be absorbed into the surface of the pipe and diffused into the steel solid phase, and applying to the heat-treated pipe material annealing treatment in the range of temperatures in vacuum, inert gas including argon gas or an atmosphere of a gas with a reducing substance including H2 gas added thereto, to result in a decrease of nitrogen concentration gradient.

Abstract: Provided is a novel high-nitrogen stainless-steel pipe which is not obtained with any conventional technique, the stainless-steel pipe having high strength, high ductility, and excellent corrosion and heat resistance and being obtained through size reduction of crystal grains and strengthening by slight plastic working besides formation of a gradient structure in which the concentration of solid-solution nitrogen continuously decreases gradually from the surface. Also provided are hollow materials of various shapes and sizes which are formed from the steel pipe and processes for producing the steel pipe and the hollow materials. An austenitic stainless-steel pipe is treated in a range of the temperatures not higher than the critical temperature for crystal grain enlargement of the steel pipe material to cause nitrogen (N) to be absorbed into the surface of the pipe and diffused into the solid phase, while minimizing the enlargement of crystal grains during the treatment.