| 摘要 |
PROBLEM TO BE SOLVED: To stably and inexpensively manufacture a high toughness and high strength thick steel plate of >=550 MPa tensile strength, free from the occurrence of shearing crack in its cutting surface even if the plate is cut by the use of a shearing machine. SOLUTION: A steel, having a composition which contains, by weight, 0.02-0.12% C, <=0.5% Si, 1.0-2.5% Mn, <=0.025% P, <=0.002% S, <=0.05% Al and <=0.006% N, and in which the P cm value determined from equation P cm=[%C]+[%Si]/30+[%Mn]/20+[%Cu]/20+[%Ni]/60+[%Cr]/20+[% Mo]/15+[%V]/10+5[%B] is regulated to 0.12-0.22, is hot rolled to thick steel plate. After rolling, the thick steel plate is subjected to accelerated cooling at >=5 deg.C/sec cooling rate. At this time, one or plural kinds selected from <=0.5% Cu, <=0.5% Ni, <=0.5% Cr, <=0.5% Mo, <=0.1% Nb, <=0.1% V, and <=0.1% Ti can be further incorporated.
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