| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel plate excellent in machinability, toughness and weldability, and to provide a method for production thereof. SOLUTION: The steel plate has a composite structure of a soft structure consisting essentially of ferrite and a hard structure consisting essentially of bainite and martensite, in which a ferrite proportion is≥30% or the region where micro-Vickers hardness is≤170 HV is≥0.2, and further, the Vickers hardness of the steel is 165 to 300 HV. The steel plate is produced by a method comprising using a steel having the contents of C, Si, Mn, P, S, Al and N being limited to prescribed ranges and optionally further containing Mo, Cr, Nb, Ti, V, Cu, Ni, B, REM, Ca, Zr and Mg, and in which X1 expressed by X1=C+(Mn+Cu+Cr)/20+Si/30+Ni/60+Mo/15+V/10+5×B is≤0.24, and X2 expressed by X2=(Si/5+Mo+Cr/2)/Mn is 0.15 to 10.0, and comprising strictly controlling the conditions in rolling, washing with water, and the like, and is excellent in machinability, toughness and weldability. COPYRIGHT: (C)2006,JPO&NCIPI
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