Ultra-high strength triple phase steels with excellent cryogenic temperature toughness

A method for preparing a triple phase steel plate having a microstructure comprising not more than about 40 vol % of a first phase of ferrite, about 50 vol % to about 90 vol % of a second phase of predominantly fine-grained lath martensite, fine-grained lower bainite, fine granular bainite (FGB), or mixtures thereof, and not more than about 10 vol % of a third phase of retained austenite, 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 T.sub.nr temperature and above about the Ar.sub.3 transformation temperature;
• (d) further reducing said steel plate in one or more hot rolling passes in a third temperature range between about the Ar.sub.3 transformation temperature and about the Ar.sub.1 transformation temperature;
• (e) quenching said steel plate at a cooling rate of at least about 10.degree. C. per second (18.degree. F./sec) to a Quench Stop Temperature below about 600.degree. C. (1110.degree. F.);
• and
• (f) stopping said quenching, said steps being performed so as to facilitate transformation of said microstructure of said steel plate to not more than about 40 vol % of a first phase of ferrite, about 50 vol % to about 90 vol % of a second phase of predominantly fine-grained lath martensite, fine-grained lower bainite, fine granular bainite (FGB), or mixtures thereof, and not more than about 10 vol % of a third phase of retained austenite.