| Summary: | Thermo-mechanical phenomena during continuous thin slab casting have been studied
with the objectives of understanding the mechanism of mold crack formation, and the
effect of mold design upon the mechanical behavior of the stand. To achieve these goals,
several finite element models have been developed in conjunction with a series of
industrial plant trials.
First, an investigation of mold crack formation in thin slab casting was undertaken to
elucidate the mechanism by which cracks develop and to evaluate possible solutions to
the problem. Three-dimensional finite-element thermal-stress models were developed to
predict temperature, distortion, and residual stress in thin-slab casting molds, comparing
funnel-shaped to parallel molds. Mold wall temperatures were obtained from POSCO in
Korea and analyzed to determine the corresponding heat-flux profiles in thin-slab molds.
This data was utilized in an elastic-visco-plastic analysis to investigate the deformation of
the molds in service for the two different mold shapes. The results of a metallurgical
investigation of mold samples containing cracks were used together with the results of
the mathematical models, to determine mechanisms and to suggest solutions for the
formation of mold cracks. Large cyclic inelastic strains were found in the funnel
transition region just below the meniscus, due to the slightly higher temperature at that
location. The cracks appear to have propagated by thermal fatigue caused by major level
fluctuations.
Next, two-dimensional thermo-elastic-visco-plastic analysis was performed for a
horizontal slice of the solidifying strand, which moves vertically down the mold during
casting. The model calculates the temperature distributions, the stresses and the strains in
the solidifying shell, and the air gap between the casting mold and the solidifying strand.
Model predictions were verified with an analytical solution and plant trials that were
carried out during billet casting at POSCO.
The validated model from the billet study was next applied to thin slab casting, using
mold temperature and distortion data from the mold cracking study. An investigation of
the effect of mold taper on the shrinkage of the solidifying shell, its gap formation, and
stress evolution was carried out for different thin slab mold geometries. The model
predicts that the shell in funnel molds develops a tensile stress at the slab surface in the
funnel transition region due to funnel retraction. This model also suggests that as the
funnel depth increases, the possibility of surface cracks at the funnel outside bed position
increases.
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