Abstract: According to quality defects such as internal crack,center shrinkage and center porosity of 37Mn5 steel round billet with high casting speed,the main reasons which lead to internal quality defects were high superheat,weak mold electromagnetic stirring intensity,unreasonable secondary cooling process system and arranging form of nozzles respectively based on analysis of production practice.Mathematical model of solidification and heat transfer verified by nailshooting experiments and surface temperature testing was applied in optimizing secondary cooling process system.Stirring parameters were optimized by measuring the electromagnetic field distribution with Gauss meter in the mold at different stirring parameters,in addition arranging form of nozzles in second cooling zone was also improved.The results showed that Φ200 mm 37Mn5 steel round billet casting speed increased from 1.4 m/min to 1.8 m/min,the internal quality significantly has been improved.Internal cracks disappeared and the average grade of center shrinkage and center porosity was 0.5 ,but when superheat was greater than 30 ℃，center porosity still existed.

Key words: 37Mn5 steel, round billet, high casting speed, internal quality