A semi-manufactured steel material has a chemical composition including, by mass %, C: 0.055% to 0.15%, Si: not more than 0.2%, Mn: not more than 1.3%, P: not more than 0.03%, S: not more than 0.007%, Al: not more than 0.1%, N: not more than 0.01%, and Ti: 0.14% to 0.30%, the balance comprising Fe and inevitable impurities. In the composition, 1.0≦([C]/12)/([Ti*]/48) is satisfied ([C], [S], [N] and [Ti]: contents (mass %) of the respective elements, and [Ti*]=[Ti]−3.4×[N]−1.5×[S]), and the contents of niobium and boron as impurities are limited to Nb: less than 0.03% and B: less than 0.0005%.