| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing 60 kilo class acceleration-cooled steel excellent in weldability and toughness after strain aging and having excellent low temp. toughness even after cold working such as bending. SOLUTION: Steel contg., by weight, 0.04 to 0.09% C, 0.1 to 0.5% Si, 1.2 to 1.8% Mn, 0.01 to 0.05% Nb, 0.002 to 0.07% sol.Al and 0.001 to 0.004% N, also satisfying Pcm<=0.20% and Ceq (WES) <=0.42% and moreover contg. one or >= two kinds among Cr, Mo, Cu, Ni, V, Ti and Ca is heated at 950 to 1,250 deg.C, is rolled at >=720 deg.C and is thereafter subjected to accelerated cooling from the temp. of 720 deg.C to Ar3 point to 400 to 650 deg.C at a cooling rate of >=2 deg.C/sec, where Pcm=C+Mn/20+Si/30+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, and Ceq (WES)=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14.
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