Abstract: Low-carbon steel and medium-carbon steel were taken as the objects of research in this paper. In order to study the effects of process parameters on the initial solidification behavior of continuously cast billets,a CA-FE coupled method was used to simulate the initial solidification behavior of billets in the mold during practical continuous casting. The dependences of initial billet shell thickness at the exit of the mold upon superheat and casting speed were investigated and the microstructure morphologies of the two steels were compared at the same time. It is shown that the shell thickness decreases with increasing superheat and casting speed,while the casting speed has a more significant impact. The shell thickness of different steels decreases at different gradients under the same condition. The lower the superheat temperature,the more compact the columnar crystals will be,which is useful to improve the quality of continuous casting billets,while the casting speed has less influence. With the constraint of shell thickness at the exit of the mold,the superheat is taken to be 15℃,and the casting speeds of low-carbon steel and medium-carbon steel could not surpass 2.2m·min^-1 and 2.5 m·min^-1,respectively. According to this result,we can design the optimum drawing speeds for different steel grades to improve the efficiency of continuous casting. Moreover,the model results show that the columnar crystals of low-carbon steel is more developed than those of medium-carbon steel.