Abstract: 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)/(SiO₂) 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%.

Key words: Steel Q345E, , Φ 800 mm Casting Bloom, Net Cracks, Depression, Mold Powder

摘要： 采用金相显微镜和扫描电镜分析了Q345E钢Φ800 mm铸坯（/%:0.15C,0.27Si,1.37Mn,0.009P,0.001S,0.03Nb,0.04V,0.030A1,0.008 ON）表面网状裂纹,得出结晶器壁和凝固坯壳之间保护渣膜厚度不均匀,使坯壳局部受挤压,产生凹坑,冷却速度降低,产生热应力裂纹。通过将保护渣碱度（CaO）/（SiO₂）从1.03提高到1.15,熔点从1 235℃降至1 210℃,1 300℃粘度从0.87 Pa·s提高到1.10 Pa·s,使Φ800 mm连铸坯表面凹坑和网状裂纹的发生率从原60.5%降至0.5%以下。