Abstract: In view of the problem of the surface longitudinal cracks on the bloom of medium carbon high manganese steel,the crack morphology was observed by SEM,the composition of the crack micro area was analyzed by energy spectrum analyzer.The precipitation temperature of AlN and TiN was calculated by thermodynamic theory according to the steel composition,and the transformation of cooling process was simulated by Thermo-Calc soft ware to provide theoretical basis of analyzing the longitudinal crack of continuous casting bloom.During the optimization process,the liquid steel was titanium alloyed,and w(Ti)=0.040 % at the end of LF.The nitrogen mass fraction of molten steel reduced to 0.005 % ~ 0.008 % by vacuum treatment.The cooling water flow rate of wide and narrow sides of mould decreased from 850，700 L/min to 750，650 L/min,respectively.And the secondary cooling water volume changed from 0.20 L/kg to 0.17 L/kg.Through the above measures,the longitudinal crack of the continuous casting slab of medium carbon high manganese steel was eliminated.

Key words: medium carbon high manganese steel, longitudinal crack, thermodynamics, phase transition, titanium alloying