| 摘要 |
PROBLEM TO BE SOLVED: To provide an automotive high tensile strength electric resistance welded steel pipe having sufficient fatigue characteristics in the electric resistance weld zone, having strength of >=50 kgf/mm2 (490 N/mm2) tensile strength and capable of stable mass-production on an industrial scale. SOLUTION: In the surface of a ferritic band in the weld zone in the cross-section in the T direction, the precipitating quantity of Ti and Nb carbonitrides of >=3.0×10-5 mm grain size per field in the area of 1.0×10-4 mm2 suppressed to <=40 pieces. The concrete steel compsn. has 0.06 to 0.30% C, <=1.0% Si, <=2.5% Mn, 0.05 to 0.8% Mo, 0.01 to 0.050% Nb, 0.005 to 0.040% Ti, 0.005 to 0.05% sol.Al, <=0.02% P, <=0.005% S and <=0.008% N, furthermore contg., at request, one or more kinds among <=1.5% Cr, <=3.0% Ni, <=1.0% Cu, 0.10% V and 0.0005 to 0.0020% B, and the balance Fe with inevitable impurities.
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