For steel and steel for the method

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PROBLEM TO BE SOLVED : To produce a steel sheet which has excellent toughness in a heat affected zone even on high heat input welding, and to provide a method for preparing molten steel used for the stock therefor. SOLUTION : The steel has composition containing, by mass, 0.01 to 0.15% C, 0.05 to 0.80% Si, 0.5 to 2.0% Mn, 0.01 to 0.06% Al, 0.003 to 0.013% rare earth elements and =10 pieces/cm , and the grain size thereof is controlled to <=500 nm.