| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel sheet excellent in shock resistance and capable of increasing energy absorption power during collision and a manufacturing method therefor.SOLUTION: There is provided a thick steel sheet having a composition containing, by mass%, C:0.05 to 0.20%, Si:0.2 to 1.0%, Mn:0.5 to 2.0%, P:0.008% or less, S:0.003% or less, Nb:0.003 to 0.030%, Al:0.002 to 0.10%, N:0.0010 to 0.0060%, O:0.0010 to 0.0060% and further one or more kind of Ca, Mg and REM of 0.0015 to 0.0080% as the total of added amount, a micro structure consisted by ferrite and a second phase mainly containing perlite and having Vickers hardness, fraction, average area S and average circumference length L of the second phase satisfying predetermined conditions and an average dislocation density in the ferrite phase of 7×10/mor less. |
