| 摘要 |
PURPOSE:To obtain a steel plate having required strength and toughness and reduced in sectional hardness difference and acoustic anisotropy with high efficiency, by subjecting a steel in which components are regulated and carbon equivalents are specified to casting and to hot rolling in the austenite recrystallization region and then controlling cooling conditions. CONSTITUTION:A steel having a composition which consists of, by weight, 0.05-0.18% C, 0.05-0.5% Si, 0.70-1.8% Mn, 0.005-0.1% Al, 0.003-0.030% Nb, <0.006% N, and the balance Fe with inevitable impurities and in which a carbon equivalent represented by C+Mn/6 is regulated to <=0.36 and a carbon equivalent represented by an expression is also regulated to <=0.40 is refined, which is cast, heated to 1,000-1,200 deg.C, and hot-rolled in the austenite recrystallization region while securing >=50% total draft, and rolling is finished at >=850 deg.C. Subsequently, cooling is applied at 5-15 deg.C/sec cooling rate from a temp. of >=(Ar3-50 deg.C) down to a temp. region of 400-680 deg.C. If necessary, one or more kinds among <=0.1% each of Ti, Zr, V, Ta and Ca and <=0.003% B are incorporated to the above steel.
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