STEEL PLATES FOR ULTRA-HIGH-STRENGTH LINEPIPES AND ULTRA-HIGH-STRENGTH LINEPIPES HAVING EXCELLENT LO

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Ultra-high-strength linepipes having excellent low-temperature toughness manufactured by welding together the edges of steel plates comprising C of 0.03 to 0.07 mass%, Si of not more than 0.6 mass%, Mn of 1.5 to 2.5 mass%, P of not more than 0.015 mass%, S of not more than 0.003 mass%, Ni of 0.1 to 1.5 mass%, Mo of 0.15 to 0.60 mass%, Nb of 0.01 to 0.10 mass%, Ti of 0.005 to 0.030 mass%, Al of not more than 0.06 mass%, one or more of required amounts of B, N, V, Cu, Cr, Ca, REM (rare-earth metals) and Mg, with the remainder consisting of iron and unavoidable impurities and having a (Hv-ave)/(Hv-M) ratio between 0.8 and 0.9 at 2.5 ≤ P ≤ 4.0, wherein Hv-ave is the average Vickers hardness in the direction of the thickness of the base metal and Hv-M is the martensite hardness depending on C-content (Hv-M = 270 + 1300C) and a tensile strength TS-C between 900 MPa and 1100 MPa; P = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45(Ni + Cu) + (1 + β)Mo - 1 + β(β = 1 when B ≥ 3 ppm and β = 0 when B < 3 ppm).