Patent ID: 7736447

Claim:
Steel plate for ultra-high-strength linepipe having excellent low-temperature toughness consisting essentially of: C 0.03 to 0.07 mass % Si not more than 0.28 mass % Mn 1.5 to 2.5 mass % P not more than 0.015 mass % S not more than 0.003 mass % Mo 0.15 to 0.60 mass % Nb 0.01 to 0.10 mass % Ti 0.005 to 0.030 mass % Al not more than 0.10 mass % and, one or more of: Ni 0.1 to 1.5 mass % B less than 3 ppm V not more than 0.10 mass % Cu not more than 1.0 mass % Cr not more than 1.0 mass % Ca not more than 0.01 mass % REM not more than 0.02 mass % Mg not more than 0.006 mass % and the remainder being iron and unavoidable impurities and having the value P defined below being between 2.5 and 4.0, and having a microstructure composed of degenerate upper bainite of more than 70%, in which; the ratio (Hv-avep)/(Hv-M) between the average Vickers hardness Hv-avep in the direction of thickness and the martensitic hardness Hv-M determined by carbon content is between 0.8 and 0.9, and the transverse tensile strength TS-Tp is between 880 MPa and 1080 MPa, P=2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+Mo−1 Hv-M=270+1300C wherein the symbols of elements designate the mass % of the individual elements.