Studies of the conditions of formation of transverse corner cracks on the surface of continuous-cast slabs

Abstract

A statistical analysis is made of production data to determine the effect of the main process parameters in the continuous casting of slabs on the probability of formation of edge cracks on the surface of the slabs. The parameters that have the greatest effect on crack formation are identified and several methods are proposed for changing these parameters to make the formation of edge cracks less likely. Corner cracks located on the surface of the casting are one type of defect commonly encountered on slabs of crack- resistant steels cast on curvilinear continuous casters. Up to 10% the slabs may be damaged by such cracks. The length of these defects ranges from several millimeters to 15-20 mm, while their depth reaches 5-10 mm. All types of surface cracks are stress concentrators that can lead to fracture of the material during rolling or the formation of different types of surface defects on flat-rolled product (1). Conditioning of the surface - hot scarfing or mechanical scarfing - is often used to remove cracks from the surface of slabs. These measures make it possible to improve the quality of the rolled product but also increase labor costs, and there is always the possibility that a defect may be missed or not completely repaired. All these con- siderations make it a matter of practical importance to re-examine the continuous-casting process in order to reduce the dam- age to slabs by corner cracks. To resolve the problem, it is necessary to make a detailed study of the conditions under which the edges of slabs are formed and cooled. It is noted in the literature that transverse cracks are formed on surface of slabs at their edges due to hanging of the ingot in the corners of the mold, the formation of shrinkage stresses in the longitudinal direction when the edges of the ingot undergo excessive cooling, or tearing of an excessively cool ingot skin during its bending or unbending (2). Thus, the condi- tions under which edge cracks are formed in the mold are related to the quality of its working surface, the effect of the slag- forming mixtures that are used, and the effect of the mold's conicity. Both of the broad surfaces of the slab are subjected to mechanical tensile stresses (the most dangerous type of stresses, leading to loss of integrity of the metal) in the secondary cooling zone (SCZ) of a radial continuous caster. The surface on the large-radius side undergoes tension as the slab is bent under the mold, while the surface on the small-radius side undergoes tension as the slab is bent in the opposite direction dur- ing its entry onto the horizontal section of the caster. It is impossible to eliminate the effects of these stresses in continuous casters of the given design. Thus, the only way to make crack formation on these sections less likely is to vary the chemical composition of the metal by changing its chemical composition and the temperature of the ingot surface. The possibilities for varying the chemical composition of the metal are severely limited by the requirements established for the given grade of