| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a steel sheet having excellent toughness in a weld heat affected zone (HAZ) and a method for manufacturing the steel sheet. <P>SOLUTION: The steel sheet having excellent toughness in a weld heat affected zone has a chemical composition which includes, by mass%, 0.02-0.2% of C, 0.03-0.5% of Si, 0.5-2.0% of Mn, 0.02% or less of P, less than 0.002% of S, 0.005-0.08% of Al, 0.003-0.02% of Ti, 0.002-0.009% of N, 0.001-0.0035% of O, and 0.0003-0.0045% of Ca with the balance being Fe and unavoidable impurities, and satisfies the following formulas (1), (2) and (3). In a microstructure in the weld heat affected zone, an area ratio of island martensite is less than 1.0%. An inclusion including Al and Ca and having a particle size of 5.0 μm or less is present in steel, and an aspect ratio of the inclusion is 1.9 or less. Further, the steel sheet may include one or more selected from among Cu, Ni, Cr, Mo, V, Nb and B. 0.50≤Ca/O≤1.30 (1), Pcm*≤0.23 (2) and Pcm*-0.75C≥0.1 (3). <P>COPYRIGHT: (C)2013,JPO&INPIT |
