LOW YIELD RATIO STEEL PLATE FOR BUILDING HAVING EXCELLENT HIGH HEAT INPUT WELD ZONE TOUGHNESS AND ME

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PROBLEM TO BE SOLVED : To provide a low yield ratio steel plate for building having high heat input weld heat affected zone toughness which has low surface hardness and excellent earthquake resistance, and to provide a method for producing the same. SOLUTION : The steel plate has a composition comprising, by mass, 0.03 to 0.07% C, 0.05 to 0.5% Si, 0.6 to 2.0% Mn, ≤0.020% P, 0.0005 to 0.003% S, 0.005 to 0.03% Ti, 0.0003 to 0.0020% B, ≤0.1% Al, 0.0025 to 0.0070% N, 0.001 to 0.003% O, 0.0005 to 0.005% Ca, ≤0.01% Mo and ≤0.005% Nb, further comprising one or more selected from Cu, Ni, Cr, V, Mg and rare earth metals, satisfying 0.40 to 0.45% Ceq and 0.2 to 0.8 ACR, and the balance Fe with inevitable impurities, has a microstructure in which ferrite fraction is 10 to 40%, and has a surface hardness of ≤350 HV10 : Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 and ACR=(Ca-(0.18+130×Ca)×O)/(1.25×S). COPYRIGHT : (C)2010, JPO&INPIT