Ultra-high strength ausaged steels with excellent cryogenic temperature toughness

A method for preparing a steel plate having a micro-laminate microstructure comprising about 2 vol % to about 10 vol % of austenite film layers and about 90 vol % to about 98 vol % laths of predominantly fine-grained martensite and fine-grained lower bainite, said method comprising the steps of:

• (a) heating a steel slab to a reheating temperature sufficiently high to (i) substantially homogenize said steel slab, (ii) dissolve substantially all carbides and carbonitrides of niobium and vanadium in said steel slab, and (iii) establish fine initial austenite grains in said steel slab;
• (b) reducing said steel slab to form steel plate in one or more hot rolling passes in a first temperature range in which austenite recrystallizes;
• (c) further reducing said steel plate in one or more hot rolling passes in a second temperature range below about the Tnr temperature and above about the Ar3 transformation temperature;
• (d) quenching said steel plate at a cooling rate of about 10° C. per second to about 40° C. per second (18° F./sec-72° F./sec) to a Quench Stop Temperature below about the Ms transformation temperature plus 100° C. (180° C.) and above about the Ms transformation temperature;
• and
• (e) stopping said quenching, so as to facilitate transformation of said steel plate to a micro-laminate microstructure of about 2 vol % to about 10 vol % of austenite film layers and about 90 vol % to about 98 vol % laths of predominantly fine-grained martensite and fine-grained lower bainite.