| 摘要 |
PURPOSE: To produce a steel for an ultrahigh strength and low yield ratio line pipe excellent in low temp. toughness and easily capable of applying field welding by forming the microstructure of a steel having a specified compsn. of low C-high Mn-Ni-Mo-Nb-a trace amt. of Ti series into a specified one. CONSTITUTION: The compsn. of a steel is constituted of the one contg., by weight, 0.05 to 0.10% C, <=0.6% Si, 1.8 to 2.5% Mn, <=0.015% P, <=0.003% S, 0.1 to 1.0% Ni, 0.35 to 0.60% Mo, 0.01 to 0.10% Nb, 0.005 to 0.030% Ti, <=0.06% Al and 0.001 to 0.006% N, furthermore contg., at need, one or more kinds among <=0.10% V, <=0.7% Cu and <=0.8% Cr or 0.001 to 0.006% Ca, and the balance iron with inevitable impurities, and in which the value of P shown by the formula is regulated to 1.9 to 2.8. Moreover, its microstructure is formed of martensite, bainite and ferrite, and in which the fractional rate of the ferrite is regulated to 20 to 90%, its average grain size to <=5μm, and the rate of the worked ferrite therein to 50 to 100%.
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