Abstract: The causes of cracks of HPB300 smallsection billet in WuKun Steel were researched and analyzed by technology investigation and using billet low magnification analysis. The results show that the main cause of billet cracks is the high sulfur content in steel (w(S)>0.030 %), reduces the critical strain value of steel；carbon content control is low (w(C)=0.12 %-0.17 %), leads to the stress concentration of weak part of billet shell, increases sensitivity of surface cracks and dents；superheat of liquid steel is higher(average 33 ℃),cooling intensity is large (the second cooling water consumption 1.92 L/kg),casting speed is slow (2.12 m/min), improve surface temperature gradient of billet, further increase crack sensitivity. For the aboved reasons, chemical composition, superheat of liquid steel, the second cooling water consumption, casting speed were optimized, quality of billet has been significantly improved, crack rate of billet was decreased from 9.5 % to 0.2 %,centre cracks completely was eliminated, corner crack and middle crack defects were dropped to grade 0.2.

Key words: HPB300 steel, crack, chemical composition, superheat of liquid steel, cooling intensity, casting speed