Abstract: The longitudinal cracks exist at the surface of heated 150 mm x 150 mm continuous casting billet of steel 55(/% ：0.52 ~0.60C,0.17 ~0.37Si,0.50 ~0.80Mn,≤ 0・035P, ≤0.035S). the subsurface crack defects of eating billet are analyzed by metallographic microscope. The result indicates that the defect is caused by the strong non-nomogneous secondary cooling and excessive harmful element Pb enriched at the grain boundaries. The optimization process of the spray system of the second cooling and reducing the harmful element Pb in steel is carried out. It improves the cooling uniformity and grain boundary strength of casting billet. The result shows the defects of casting billet have been effective controlled. The qualification rate of the first inspection of rolled products increases from 45% to 93%.

摘要： 55钢(/%:0.52～0.60C,0.17～0.37Si,0.50～0.80Mn,≤0.035P,≤0.035S)的150 mm×150 mm连铸坯轧钢加热炉加热后存在表面纵向裂纹缺陷。采用金相显微镜对铸坯皮下裂纹缺陷进行分析,结果得出:由于二次冷却不均匀和有害元素Pb在晶界富集导致铸坯皮下产生细小裂纹并扩展长大。通过对二次冷却喷淋系统优化及降低钢水有害元素Pb含量,改善二冷段喷淋冷却效果,提高铸坯冷却均匀性,提高铸坯晶界强度,结果表明:铸坯缺陷明显改善,轧材一次探伤合格率从45%提高到93%。