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 of the continuous casting process are reviewed, focusing on treatment of the mushy zone and hot-tear criteria. Existing hot-tear criteria are reviewed and evaluated. A two-phase (solid and liquid) model describing the mushy steel is developed as an initial step toward developing comprehensive hot tear criteria for steel casting. An empirical strain criterion is, then, chosen to predict hot tear cracks, based on thermal and stress histories. A coupled finite-element model, CON2D, is 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 superheat 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. Firstly, the minimum shell thickness to avoid breakouts is predicted. Failure occurs v when the total strain across the local thin shell due to ferrostatic pressure exceeds the critical strain where hot tear cracks initiate. The predicted minimum shell thickness is only 3mm and varies slightly with steel carbon content. Without local thin spots, this restriction does not put practical limits on maximum casting speed. Considering that peritectic steels are more likely to have local thin spots, this explains their propensity for breakouts. Secondly, the maximum casting speed to avoid hot off-corner sub-surface cracks is predicted when the maximum accumulated damage strain exceeds the critical strain. This first occurs beneath off-corner surface. The predicted casting speed limit is up to 6.4m/min for a 120mm square billet casting in a 1100mm long mold. The limit drops with increasing section size and decreasing mold length. These predictions are confirmed by reported plant practice casting speeds. Thirdly, optimal mold taper is investigated along the casting direction with special attention to the corner effects. Simulation shows that the mold wall at the billet corner should not exactly follow the strand as commonly believed. Instead, a small, controlled amount of gap at the corner is preferred to produce uniform surface temperature in order to avoid both in-mold corner surface cracks and sub-mold off-corner sub-surface cracks.