A Formation Mechanism of Transverse Cracks on CC Slab Surface

概要

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Transverse surface cracks were metallographically investigated and the local segregation was found to be the origin of transverse surface cracks on continuously cast slabs. The positive segregation spreaded over an oscillation mark and showed high phosphorous content. At the same time, the sharp negative segregation was also seen adjacent to the positive segregation along the mark. This positive segregation penetrated deeply into austenite grain boundaries, through which transverse cracks propagated. The fractured surface was smooth and the precipitates of MnS was found.Based on the observation, a mechanism of the formation of cracks was proposed. At the meniscus, when the tip of partially solidified shell is bent down periodically during mould oscillation, interdendritic "dirty" liquid is squeezed out and accumulated at the valley of an oscillation mark. In other case, bulk liquid overflows on the tip of the shell and again interdendritic "dirty" liquid spreads out and accumulates there. In any case, the local segregation forms at the valley of an oscillation mark. The cracks lead to initiate at this weak part of the shell under any external stresses. Thus so called internal cracks forms below the oscillation mark and propagates along austenite grain boundaries. Finally a large transverse crack forms on the above-mentioned defect. Some industrial measure were summarized to prevent both the segregation and the cracks to form.