ULTRA-HIGH STRENGTH, WELDABLE STEELS WITH EXCELLENT ULTRA-LOW TEMPERATURE TOUGHNESS

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A steel plate having a tensile strength of at least about 930 MPa (135Ksi), a toughness as measured by Charpy V-notch impact test at -60 C (-76 F) of at least about 120 joules (88 ft-lb), and a microstructure comprising at least about 90 volume percent of a mixture of fine-grained lower bainite and fine-grained lath martensite, wherein at least about 2/3 of said mixture consists of fine-grained lower bainite transformed from unrecrystallized austenite having an average grain size of less than about microns and comprising iron and specified weight percentages of the additives : carbon, silicon, manganese, copper, nickel, niobium, titanium, aluminum, calcium, Rare Earth Metals, and magnesium, is prepared by heating a steel slab to a suitable temperature; reducing the slab to form plate in one or more hot rolling passes (10) in a first temperature range in which austenite recrystallizes; further reducing said plate in one or more hot rolling passes (10) in a second temperature range in which austenite does not recrystallize, quenching (12) said plate to a suitable Quench Stop Temperature (16); and stopping said quenching and allowing said plate to air cool (18) to ambient temperature.