Steel with improved impact penetration resistance and method for producing the same

1. Field of the Invention A steel has a tensile strength of 850 to 1700 MPa, a yield ratio of at most 80%, and a penetration border energy ratio of at least 2.0 relative to the penetration border energy of a reference steel JIS SS400 (corresponding to ASTM A36) of the same thickness. The steel can be produced by first effecting a heat treatment on an unstable austenitic steel; then effecting, at least once, one or more or any combination of the heat treatment and a heat treatment on the steel; and then finally effecting the heat treatment on the steel. Heat treatment heats the unstable austenitic steel to at least the Ac3 transformation temperature and then water-cools the steel to a temperature below 350° C. Heat treatment heats the steel to a temperature between Ac3 and Ac1 transformation temperatures and then water-cools the steel to a temperature below 350° C.