STEEL SHEET EXCELLENT IN SHOCK RESISTANCE AND MANUFACTURING METHOD THEREFOR

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. 【課題】衝突時のエネルギー吸収能を増加させることが可能な、耐衝撃特性に優れた鋼板及びその製造方法を提供する。【解決手段】質量％で、Ｃ：０．０５〜０．２０％、Ｓｉ：０．２〜１．０％、Ｍｎ：０．５〜２．０％、Ｐ：０．００８％以下、Ｓ：０．００３％以下、Ｎｂ：０．００３〜０．０３０％、Ａｌ：０．００２〜０．１０％、Ｎ：０．００１０〜０．００６０％、Ｏ：０．００１０〜０．００６０％を含有し、さらにＣａ、Ｍｇ、ＲＥＭの１種または２種以上を添加量の合計として０．００１５〜０．００８０％含有する組成であって、ミクロ組織がフェライト、及びパーライトを主体とする第二相から構成され、該第二相のビッカース硬さ、分率、平均面積Ｓ、平均周囲長Ｌが所定の条件を満たすとともに、フェライト相中の平均転位密度が７?１０１２／ｍ２以下である厚鋼板。【選択図】 図１