The surface net cracks of Φ 800 mm casting bloom of steel Q345E (/% : 0.15C

0.27Si

1. 37Mn

0. 009P

0. 001S

0. 03Nb

0. 04V

0. 030Al

0.008 ON) have been analyzed by metallurgy and scanning electron microscope. It is obtained that the uneven thickness of flux film in between mold wall and solid shell leads to extruding local shell

forming depression

decreasing cooling rate and forming thermal stress cracks. With increasing basicity of mold powder (CaO)/(SiO

2

) from 1. 03 to 1.15

decreasing melting point form 1 235 °C to 1 210 °C

and increasing viscosity at 1 300 °C from 0. 87 Pa · s to 1. 10 Pa· s

the occurring rate of surface depression and net cracks of 0800 mm casting bloom decreases from original 60. 5% to less than 0. 5%.