Manufacturing method of welding crack susceptibility and low-temperature toughness in excellent tone quality type 600N / mm2 class high tensile steel

摘要

PROBLEM TO BE SOLVED: To provide a method for producing a quenched and tempered 60 kilo class steel with ≤50mm plate thickness excellent in both characteristics of weld crack sensitivity and low temp. toughness without executing controlled rolling at a low temp. ;SOLUTION: A steel having a compsn. contg., by weight, 0.04 to 0.1% C, 0.01 to 0.4% Si, 0.5 to 1.6% Mn, ≤0.015% P, ≤0.01% S, 0.005 to 0.05% Nb, ≤0.1% V, 0.005 to 0.1% Al, 0.0005 to 0.008% N, 0.005% Ti and 0.0003% B, in which Pcm value defined by Pcm=C+Si/30+Mn/20+Cu/20+Ni/30+Cr/20+Mo/15+V/10+5B is regulated to ≤0.2, and the balance iron with inevitable impurities is heated to the temp. range of 1000 to 1250°C in the case of hot rolling, is subjected to cumulative rolling reduction of ≥20% at 1000 to 1050°C, is successively subjected to cumulative rolling reduction of ≥20% at 1000 to ≥800°C, is directly quenched from at least the Ar₃ transformation point of above and is thereafter tempered at the Ac₁ transformation point or below. This steel has ≥570N/mm² tensile strength and is excellent in weld crack sensitivity and low temp. toughness.;COPYRIGHT: (C)1998,JPO