[Mn]/[S] and process factors including superheating degree of liquid on center cracks of 160 mm x 200 mm rectangular billet of 45C Steel (0. 44% 〜0. 50% C) has been classified
counted and analyzed. According to analysis results the process measures including controlling [P]≤0. 020%
[S]≤015%
[ Mn]/ [S]≥40
superheating degree ≤25
and optimizing secondary cooling water rate with casting speed 1. 8 m/min are a- dopted
as a result the occurring rate of center cracks decreases from original 82. 76% to 22. 41% and that more than rating 1 decreases from original 22. 41% to 0. 51% .