| 摘要 |
PROBLEM TO BE SOLVED: To obtain excellent immunity to impact deformation by specifying a steel composition and performing, after hot rolling, controlled cooling and structure control. SOLUTION: This steel plate has a composition consisting of, by weight ratio, 0.05-2.0% C, <=1.5% Si, 0.5-3.0% Mn, <=0.01% S, further one or >=2 kinds among 0.03-0.15% P, 0.05-2.0% Cr, and 0.1-1% Mo, and the balance Fe with inevitable impurities. Simultaneously, the surface layer part, between the surface of the steel plate and a position at a depth of at least 2% of plate thickness, has a dual-phase structure of ferrite of <5μm average grain size and martensite of <3μm average grain size. At the time of producing the steel plate, the steel is heated to 950-1250 deg.C, roughed, subjected to ultra high pressure descaling at >=25kgf/cm<2> pressure of collision, and finished at a temp. not lower than the Ar3 transformation point. Subsequently, the resultant steel plate is cooled to the range of 750-700 deg.C at a rate of >=30 deg.C/S, then rapidly cooled down to 650-550 deg.C, thereafter cooled down to 500 deg.C at a rate of >=30 deg.C/S, and coiled.
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