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due to various types of surface defects, i.e. transverse and longitudinal cracks and surface entrapments from mould Rolled plates from continuously cast slabs at Bhilai powder origin. Of these, transverse cracking was of major Steel Plant have been frequently rejected due to the concern, since the average rejection of plates due to this was formation of irregular shaped fine surface cracks. more than 4%. These cracks used to appear as fine irregular These cracks originate from fine transverse cracks shaped surface cracks on the plates. Minor scarfing on the located below oscillation marks/surface depressions slab surface also revealed this defect. This clearly indicated of the cast slabs. The random occurrence of this that the origin of the transverse cracks was primarily from defect from heat to heat was of major concern to continuous casting. The principal features of this defect Bhilai Steel Plant where slabs are cast of special were: grades such as boiler, microalloying, and high (i) there is a wide variation in rejection level from heat tensile quality. The mechanism of formation of this to heat defect was identified through a detailed (ii) a high rejection heat is preceded or succeeded by metallographic study of defect bearing slabs and several zero rejection heats in the same sequence plates and analysis of rejection patterns with (iii) the occurrence of cracks were random in nature. respect to various important process parameters.Various investigators1-3 have outlined the reasons for trans-The crack formation mechanism was verified verse crack formation during continuous casting of slab and through a process perturbation trial. A defect control measures. The occurrence of transverse cracks could potential index was developed to explain the be both on the narrow side and/or at the corner of slabs.4,5 random occurrence of these cracks. The index Cracks may also be seen below the surface depression.6 incorporated parameters which influence crackThese cracks are perpendicular to the casting direction, and formation in the Bhilai casters. A significant can be seen below the oscillation marks. In general, transreduction in this type of cracking was observed, verse cracks form on the inner side of the strand surface in based on the recommendations made in this study a curved continuous caster having a single unbending point I&S/1559 between the curved region and straightening rolls.1 The above observations confirmed that the formation of trans-

Fig. 1.13 Melting under OsciHationThe experimental rig used for melting rate measurement under oscillation is shown In Fig. 3. 14

Mould heat transfer in continuous casting has profound influence on caster operation and product quality. The average heat flux through the mould wall can be calculated from the mould water flow and the rise in temperature of the cooling water. However, the local heat flux in the mould wall gives a better understanding of the heat transfer in mould vis‐à‐vis the operating parameters. In the present investigation, all the 4 sides of a slab mould are instrumented with thermocouples in order to determine the axial temperature profiles under wide range of casting conditions. A heat flow model of the mould wall has been developed to compute the local heat flux and the hot face temperature of the mould from the measured mould temperature data. The local heat flux in the mould is correlated with the operating variables of the caster.

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