A plant experiment was carried out the copper mould of a continuous slab caster was artificially grooved and thus longitudinal mid-face cracks were formed. The obtained cast samples were metallographically examined. Local segregation of C, Mn, P and S existed in front of longitudinal cracks in both directions of thickness and length and the cracks propagated along these segregated regions. This feature was common in cracks appeared differently.
The very vicinity to the crack entry showed the deformation of ferrite and pearlite structure. This means that the opening of cracks to the surface occurred when the surface temperature decreased below Ar3 point, possibly in the secondary cooling zone.
A formation mechanism of longitudinal surface cracks was proposed based on these findings.
