To obtain a steel tube excellent in the toughness of the heat-affected zone and free from the generation of transverse cracking at the time of welding by allowing weld metal in a welded steel tube contg. one or more kinds among 15 kinds of elements by specified ratios, in which the ratios lie in specified relation to contain similar elements by specified ratios, in which the ratios lie in specified relation, and the ratio of Al is controlled to the specified one.
The welded steel tube has a compsn. contg., by weight, 0.02 to 0.15% C, ≤0.6% Si, 0.2 to 2.5% Mn, ≤0.015% P, ≤0.003% S, 0.2 to 3.5% Ni, ≤0.1% Sol.Al, 0.0005 to 0.005% Ca and one or more kinds among 0.01 to 0.1% Nb 0.01 to 0.1% Ti, 0.0005 to 0.0025% B, 0.01 to 1.5% Cu, 0.01 to 1% Cr, 0.01 to 0.6% Mo and 0.01 to.01% V, in which P cm in formula I is ≥0.18%, and its tensile strength is ≥800 MPa. The weld metal has a compsn. contg. 0.01 to 0.15% C, 0.02 to 0.6% Si, 0.6 to 3% Mn, >3 to 9.5% Ni, 0.004 to 0.08% Al, 0.003 to 0.03% Ti and ≤0.06% O, in which P cm in formula I is ≥0.25%, and, it satisfies formula II.
KOMIZO YUICHI
OKAGUCHI HIDEJI
FUJIWARA TOMOYA
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