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. Thus, a gradient structure is formed, the structure including a part that is close to the surface part of the pipe and has been highly strengthened by the formation of a high-concentration
solid solution of N and a part in which
ductility gradually increases toward around the center of the cross-section of the pipe as the N concentration decreases. Thereafter, the pipe is subjected to
size reduction of crystal grains by utilizing, for example, eutectoid transformation of the
austenite phase, thereby greatly improving the elongation (
ductility) of the steel pipe. Furthermore, the steel pipe is strengthened by slight plastic working to give a high-nitrogen austenitic stainless-steel pipe having high strength, high ductility, and excellent
corrosion and
heat resistance. A plurality of the thus-obtained high-nitrogen
austenitic stainless steel pipes of the same quality are disposed one over another so as to result in dimensions, e.g.,
diameter and wall thickness, according to the use or strength level, and this pipe arrangement is united by adhesion
processing through hot drawing, hot rolling, or other method to give a high-nitrogen austenitic stainless-steel pipe or hollow material which has high strength, high ductility, and
heat resistance and has repetitions of the gradient structure within the wall. This stainless-steel pipe or hollow material can have large or small sizes and be of various kinds, and examples thereof include a hollow material for use as a container for storing high-pressure
hydrogen gas which is for fuel
cell vehicles (FCVs) and which does not suffer
hydrogen gas
embrittlement (HGE) in the presence of high-pressure
hydrogen gas.