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AssigneeExxonMobile Upstream Research Company
InventorsJayoung Koo, Narasimha-Rao V. Bangaru, Glen A. Vaughn, Raghavan Ayer
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An ultra-high strength, weldable, low alloy steel with excellent cryogenic temperature toughness in the base plate and in the heat affected zone (HAZ) when welded, having a tensile strength greater than about 830 MPa (120 ksi) and a microstructure comprising (i) predominantly fine-grained lower bainite, fine-grained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) up to about 10 vol % retained austenite, is prepared by heating a steel slab comprising iron and specified weight percentages of some or all of the additives carbon, manganese, nickel, nitrogen, copper, chromium, molybdenum, silicon, niobium, vanadium, titanium, aluminum, and boron; reducing the slab to form plate in one or more passes in a temperature range in which austenite recrystallizes; finish rolling the plate in one or more passes in a temperature range below the austenite recrystallization temperature and above the Ar3 transformation temperature; quenching the finish rolled plate to a suitable Quench Stop Temperature (QST); stopping the quenching; and either, for a period of time, holding the plate substantially isothermally at the QST or slow-cooling the plate before air cooling, or simply air cooling the plate to ambient temperature.
A method for preparing a steel plate having a microstructure comprising (i) predominantly fine-grained lower bainite, fine-grained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) >0 to about 10 vol % retained austenite, said method comprising the steps of:
A steel plate having a microstructure comprising (i) predominantly fine-grained lower bainite, fine-grained lath martensite, fine granular bainite (FGB), or mixtures thereof, and (ii) >0 to about 10 vol % retained austenite, having a tensile strength greater than about 830 MPa (120 ksi), and having a DBTT of lower than about −62° C. (−80° F.) in both said steel plate and its HAZ, and wherein said steel plate is produced from a reheated steel slab comprising iron and the following alloying elements in the weight percents indicated:
A method for enhancing the crack propagation resistance of a steel plate, said method comprising processing said steel plate to produce a predominantly micro-laminate microstructure comprising laths of fine-grained lath martensite, laths of fine-grained lower bainite, or mixtures thereof, and >0 to about 10 vol % retained austenite film layers, said micro-laminate microstructure being optimized to substantially maximize crack path tortuosity by thermo-mechanical controlled rolling processing that provides a plurality of high angle interfaces between said laths of fine-grained martensite and fine-grained lower bainite and said retained austenite film layers.
A method for enhancing the crack propagation resistance of a steel plate, said method comprising processing said steel plate to produce a microstructure of predominantly fine granular bainite (FGB), wherein said fine granular bainite (FGB) comprises bainitic ferrite grains and particles of mixtures of martensite and retained austenite, and wherein said microstructure is optimized to substantially maximize crack path tortuosity by thermo-mechanical controlled rolling processing that provides a plurality of high angle interfaces between said bainitic ferrite grains and between said bainitic ferrite grains and said particles of mixtures of martensite and retained austenite.
Dependencies are inferred from the claim text.