A mechanism of longitudinal surface crack formation in the continuously cast slab of the medium carbon steel

摘要

A mechanism of longitudinal surface crackformation in the continuous casting mold for the peritecticmedium carbon steel was investigated by measurement of thefracture strength, the heat flux and the shrinkage of solidifyingshell. The fracture strength of medium carbon steel shell is higherthan that of other carbon steels and it exhibits an anomalousdecrease of the heat flux on the mold surface due to the surfaceroughness of the solidifying shell caused by the δ?γ phasetransformation just after solidification. Because of the largershrinkage and higher strength, the peritectic medium carbon steelshell forms larger air gap between the shell surface and the moldwall, resulting decrease of the heat flux to the mold and hencehigher recalescent shell temperature. The recalescence of the shellwill induce the crack in the sub-surface at low ductileinterdendritic area or grain boundary of the solidifying shell ifcombined with the stress due to the bending and frictional force.The sub-surface crack will propagate to the shell surface via grainboundraries at a later stage because of thermal and mechanical tensile stresses.