| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of manufacture of a thick-walled 780N/mm<2> class steel excellent in weldability, acoustic anisotropy, and toughness in the central part of plate thickness. SOLUTION: A steel, which has a composition consisting of, by weight, 0.055-0.09% C, 0.5-1.5% Mn, <=0.01% P, <=0.01% S, 0.5-3.0% Ni, 0.1-0.8% Mo, 0.003-0.3% Nb, 0.01-0.08% Al, 0.001-0.008% N, and the balance iron, satisfying Ceq=(C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14)>=0.48, and practically free from B, is used. This steel is heated to 1000-1200 deg.C, and hot rolling is finished at a steel plate surface temp. between TF and 1150 deg.C, where TF=[20Mn+10Ni+15Cr+100Mo+1500Nb+15Cu+150V+500(Ti-3.42N)-8√(plate thickness) (mm)+830] is satisfied. At this time, draft is regulated toε>=1.5×10<3>×exp(-5.5×10<-3>×T)% per pass with respect to temp. T deg.C at 1/2t plate thickness, and, after the completion of rolling, the resultant plate is directly hardened from a temp. not lower than the Ar3 transformation point and them tempered to a temp. not higher than the Ac1 transformation point.
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