Thermal-mechanical model of solidifying steel shell behavior and its applications in high speed continuous casting of billets

Hot tear cracks and related defects are important issues that limit both productivity and quality of the continuous casting of steels. Computational models can play a crucial role for understanding and predicting these problems, and providing directions for improvements. Previous thermal stress models for the continuous casting process are reviewed, focusing on treatment of the mushy zone. Existing hot tear criteria are reviewed and evaluated. Recent RDB hot tear criteria for aluminum DC casting are limited from application to steel casting by the lack of experiment data and mathematical models addressing the mechanical behavior of mushy steels. A two-phase (solid and liquid) model for steel is developed as an initial step toward developing comprehensive hot tear criteria for continuous casting of steels. An empirical strain criterion is applied to predict hot tear cracks, based on thermal and stress histories.; A coupled finite-element model, CON2D, has been improved to predict hot tear cracks based on the temperature and stress histories during the continuous casting of steel focusing on high speed billet casting. Thermal boundary conditions are investigated to make realistic predictions of high-speed casting. These include heat flow at the strand surface, gap dependent thermal model across the interfacial layer between the mold and steel strand, and uneven super heat distribution at the solidification front due to the flow of liquid steel. A method based on a micro-segregation model is implemented to provide better liquid and solid phase fractions. A creep based constitutive model is applied to treat liquid and mushy regions, rather than using a non-physical small elastic modulus as done in previous models. The empirical hot tear criterion is integrated into CON2D to predict hot tear cracks. The model is first validated by accurately matching an analytical solution for both temperature and stress in a solidifying slab with properly refined mesh and time step sizes. It is further validated by simulating continuous casting of a 120mm billet and compares favorably with plant measurements of mold wall temperature, total heat removal, shell thickness, including thinning of the corner and bulged shape.; The model is then applied to investigate three separate issues in high speed continuous casting: the minimum shell thickness to avoid breakouts, the maximum casting speed to avoid hot off-corner sub-surface cracks, and the optimal mold taper.