후강판 및 그의 제조 방법

성분 조성이, C: 0.02∼0.10질량%, Si: 0.10∼0.60질량%, Mn: 1.00∼2.00질량%, P: 0질량% 초과 0.035질량% 이하, S: 0질량% 초과 0.035질량% 이하, Cu: 0.10∼0.60질량%, Al: 0.010∼0.060질량%, Nb: 0질량% 초과 0.050질량% 이하, Ti: 0질량% 초과 0.050질량% 이하, N: 0.0010∼0.0100질량%, 및 잔부: 철 및 불가피 불순물로 이루어지고, 또한 Si 및 Cu의 합계 함유량이 0.30질량% 이상이며, 금속 조직이, MA 분율이 0.5면적% 이하, 및 200μm 사방의 영역에 관찰되는 소경각 입계의 총길이가 2.5mm 이상을 만족시키는 후강판. A thick steel sheet which has a component composition that contains from 0.02% by mass to 0.10% by mass of C, from 0.10% by mass to 0.60% by mass of Si, from 1.00% by mass to 2.00% by mass of Mn, more than 0% by mass but not more than 0.035% by mass of P, more than 0% by mass but not more than 0.035% by mass of S, from 0.10% by mass to 0.60% by mass of Cu, from 0.010% by mass to 0.060% by mass of Al, more than 0% by mass but not more than 0.050% by mass of Nb, more than 0% by mass but not more than 0.050% by mass of Ti, from 0.0010% by mass to 0.0100% by mass of N, with the balance being made up of iron and unavoidable impurities. With respect to this thick steel sheet, the total content of Si and Cu is 0.30% by mass or more, and the metal structure thereof has an MA fraction of 0.5% by area or less and a total length of small tilt angle grain boundaries of 2.5 mm or more in a 200 μm square area.