PURPOSE: A Cr-Ni-Mn based steel consisting of austenite phase is used as raw material which is treated for solid-solutionizing followed by cold processing, thereby to provide a strength greater than 120kg/mm2, an excellent antidelayed breakability and high toughness.

CONSTITUTION: A Cr-Ni-Mn based steel consists of Cr equivalent about 18% as (Cr%+Mo%+1.5×Si%+0.5×Nb%), Mn content 5 to 20%, Ni being reduced as much as possible to Ni equvalent 2 to 12% as (Ni%+0.5×Mn%+30×C%+ 30×N%). In order to increase steel strength and to improve delayed breakage property, C and N are solid-solutioned as much as possible within a range of solid solution N<0.25%. Such steel consisting of austenite phase is treated for solutblizing at 1050°C for one hour followed by cold processing of high reduction 70% or more, providing a high quality equivalent to maraging steel in hardeness, tensile strength, and anti-delayed breakage properties.