| 摘要 |
PROBLEM TO BE SOLVED: To improve the toughness in the surface layer part of a steel plate, by specifying its compsn. and the heating rate at the time of quenching and imparting a high dislocation density to the structure at the time of quenching to form the structure into a fine martensite-bainite mixed structure. SOLUTION: The compsn. of the steel plate contain, by weight, 0.02 to 0.15% C, <=0.3% Si, 0.4 to 2% Mn, 0 to 0.6% Cu, 1 to 6% Ni, 0 to 0.8% Cr, 0.3 to 2% Mo, 0.01 to 0.05% Nb, 0 to 0.08% V, 0.001 to 0.005% B, 0 to 0.03% Ti, <=0.08% Al, <=0.006% N, and the balance substantial Fe. Simultaneously, theγtransforming temp. T1 and transition stabilizing temp. T2 shown by the formulae I and II satisfy the relation of T1<=T2, and the structure of the surface layer part in the steel plate is formed of a martensite-bainite structure having <=20μm averageγgrain diameter and 15μm average bucket diameter. Moreover, as for the objective mechanical properties, TS: >=780MPa, Ys: >=690MPa and Vts: <=-70 deg.C are regulated.
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