Star cracks in continuously cast peritectic steel slabs

Abstract

This work consisted of the characterisation of star cracks found on the surface of continuously cast steel slabs solidification experiments on molten steel in Cu moulds, with and without ceramic insulation. Cracks located at the slab surface were propagated through the grain boundaries, with the presence of Cu particles observed along the crack paths. Zones normal to the cracks exhibited an iron oxide layer followed by Cu particles and the steel matrix. Specimens solidified in Cu moulds showed an iron oxide/Cu/steel layer arrangement extending from the sample surface towards the steel matrix, as well as a penetration of Cu through the grain boundaries. In contrast, in samples solidified in ceramic insulated Cu moulds, Cu particles were neither observed at the surface nor at the grain boundaries. Thus, it is thought that the star cracks are due to liquid steel/Cu mould interaction during casting and that they are formed during primary cooling in the mould in the continuous casting machine and then they propagate during secondary cooling and bending operations. It is also thought that cracking susceptibility is promoted by peritectic transformation which occurs at high solid fractions.