It is obtained by analysis that the surface defects of the bars are formed before heating the continuous casting billet and related to the mold meniscus fluxes. Thermo-Calc software is used to calculate the solidification phase transition of 15CrMoG steel and the formation mechanism of surface defects for 15CrMoG steel bars are analyzed. The occurrence of peritectic reaction( L+δ→γ) and peritectic transformation ( δ→γ) near the solidus temperature of 15CrMoG steels lead to the uneven growth of the initial solidified shell and intensified the segregation of P and S elements at the solidification front. Uneven growth of the initial solidified shell is the primary cause of surface defects for the bars. Fine longitudinal cracks is generated in

the weakness of the shell

and grow in the place where inclusions and sulphur gathered. Surface clefts is caused by slag inclusions in casting blanks which caused by mold level fluctuation rooted in uneven growth of the initial solidified shell. By the measures including to control [C

]

0.16% ~0.17%

[S

]

≤0.005%

shielding slag basicity 1.2

slag melting point 1200℃ and viscosity 1.0Pa·s

water rate of 260 mm x 300 mm cast bloom 150 m

3

/h

and casting speed 0. 5 m/min

the non -destructive test qualified rate increases from original 45% to 98% .