Periodic mould level fluctuation(MLF) during slab casting is a bottleneck for upgrading the surface quality and casting speed especially for hypoperitectic(HP) or ultralow carbon steels.The uneven growth of the initially solidified shell is verified to be one of the important inducements to MLF due to related unsteady bulging in the secondary cooling zone.It is shown that the solidification mode of steels and the contraction behavior can be modified through chemical composition optimization within given composition limits.For high strength low alloy(HSLA) steels,the actual peritectic points calculated by Thermo-Calc software may change remarkably with the slight variations of alloying element contents.Accordingly,the narrow limit of chemical composition of HP steels through optimization is proven to be one of the effective factors to control the popular MLF phenomenon during slab casting. Periodic mold level fluctuation (MLF) during slab casting is a bottleneck for upgrading the surface quality and casting speed for hypoperitectic (HP) or ultralow carbon steels. The uneven growth of the initially solidified shell is verified to be one of the important inducements to MLF due to related unsteady bulging in the secondary cooling zone. It is shown that the solidification mode of steels and the contraction behavior can be modified through chemical composition optimization within given composition limits. For high strength low alloy (HSLA) steels, the actual peritectic points calculated by Thermo-Calc software may change remarkably with the slight variations of alloying element contents. Accreditedly, the narrow limit of chemical composition of HP steels through optimization is proven to be one of the effective factors to control the popular MLF phenomenon during slab casting .