UBC Theses and Dissertations
Mathematical modelling of the unbending of continuously cast steel slabs Uehara, Masatsugu
A two-dimensional, elasto-plastic, finite-element model has been developed to calculate the bending and bulging deformation of a partially solidified continuously cast steel slab during straightening on a curved-mould casting machine. A preliminary, three-dimensional elastic analysis revealed that a two-dimensional plane-stress model is sufficient for the calculations. The effects of solid-shell motion have been considered in part by shifting the roll points in two steps. The model was checked by comparing predictions of internal cracks with plant data. From the results of calculations of a one-point bending bow-type caster (10.5 m radius) for casting speeds of 1.0, 1.2 and 1.6 m/min, it has been verified that internal cracks appear at the solidification front in the upper shell due to straightening of the strand at the higher casting speeds. The critical strain for internal cracks was taken to be 0.25-0.3% at a strain rate of 1x10⁻⁴ s⁻¹ for low-carbon steels. It has been found that the upper and lower shells deform separately around their individual neutral axes, which are shifted to within 15 mm of the respective solidification fronts by the roll-friction force. Therefore the bending strain, ε[sub u],in the low-ductility region close to the solidification front can be very small, lower by about 0.3% than the value predicted by one neutral-axis theory. However, as a result of the interaction with the bulging strain, ε[sub B], the resultant total strain, ε[sub T], becomes large enough to cause internal cracks(radial streaks) close to the solidification front of the upper shell. The correlation among these variables is as follows; ε[sub T] = (2-5) ε[sub B] + ε[sub u]. Thus, the bulging strain affects the total strain significantly; and to prevent internal cracks it is important to suppress the bulging by having low surface temperatures and small roll pitches during straightening. By comparing machine radii of 8 m, 10.5 m and 13 m for a one-point bending bow-type caster, it has been verified that the small machine radius of 8.0 m is unfavorable because at normal casting speeds the tensile strain at the solidification front exceeds the critical value for crack formation.
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