Mechanisms of longitudinal depression formation in steel continuous casting
|Thomas, Brian G.
|Zappulla, Matthew L. S.
|Includes bibliographical references.
|Embargo removed at author's request on 07/21/2020.
|In continuous casting, longitudinal cracks and depressions are some of the most dangerous defects in the cast product. Cracks and depressions can lead to substantial mitigation costs or sudden failures in the final product, and in the worst case can lead to catastrophic breakouts in an operating caster.This work develops and applies computational models to examine and elucidate the behavior and formation mechanisms for longitudinal depression defects to relate the shape of the defect to the mechanism that caused it. The primary tools used are heat transfer solidification models coupled with a temperature-, phase-, and strain-rate-dependent, elastic-viscoplastic thermal-mechanical finite element model, including a new crack propagation methodology implemented. When possible, industrial samples and examples of the defects discussed are shown and used as qualitative and quantitative validation of the models. Longitudinal depressions in continuous casting can occur in both compression and tension conditions. Tension conditions result in partial necking of the steel shell and generally lead to the most common type of observed depression: a U-shape. In certain conditions, thinning of the shell is severe enough to allow the ferrostatic pressure to partially bend the depression back towards the mold and create a W-shape depression. Depressions can also form in severe compression, due to a partial inelastic buckling (or kneeling) of the shell, also leading to a W-shape depression.Depressions often contain cracks, and depressions with cracks tend to be deeper and more severe in nature than their non-cracked counterparts. Based on model predictions, it has been found that the presence and growth of a crack within a depression tends to deepen a depression, thereby decreasing its width/depth aspect ratio. Additionally, it has been observed that crack proximity to the solidification front, as well as thermal disruption proximity to the meniscus, is directly related to an increase in defect severity. As a result, it seems that many of the serious crack and depression defects commonly found in cast slabs start very close to the meniscus (as a result of some thermal disruption event) and grow along with the solidification front.Furthermore, it has also been found that the initiation location, and the direction of crack growth, changes the appearance of the depression. The presence of a surface crack(s) is observed to distribute tensile stresses that would otherwise go into further necking of the depression. It has also been found that for depressions without a surface crack, there is a tendency to develop excess sub-surface tension and thus sub-surface hot tear macro-segregation streaks.Additionally, cracks that start sub-surface and propagate towards the surface of the steel shell are found to adopt a shape where the surface sharply changes slope towards the base of the depression. In cases where the crack completely traveled to the surface, the corners of the crack (often thin and pointed) point away from the surface as the thin ligament gave away.Finally, a novel methodology has been developed for real-time prediction of longitudinal depressions and defects using integrated mold temperature measurements mapped onto the surface of the as-cast slab. This methodology can be used to augment existing breakout detection systems by taking high resolution temperature data histories of discrete points on the surface of the slab to predict longitudinal bleeder type breakouts.
|Colorado School of Mines. Arthur Lakes Library
|2020 - Mines Theses & Dissertations
|Copyright of the original work is retained by the author.
|finite element model
|Mechanisms of longitudinal depression formation in steel continuous casting
|Berger, John R.
|Findley, Kip Owen
|Doctor of Philosophy (Ph.D.)
|Colorado School of Mines