Grade effects on thermal-mechanical behavior during the initial solidification of steel
This work takes initial steps towards the ultimate goal of fundamental understanding of how cracks and depressions form during continuous casting of steel, and demonstrates a tool towards troubleshooting and preventing them. Mathematical models are developed and applied to examine thermal and mechanical behavior in the mold region of the continuous casting process. First, the models are use to study the effect of changes in steel grade early in the process for uniform solidification. Next, a thermal resistor model was developed of the interfacial gap, and finally, these two models are combined for a preliminary coupled analysis of a depression. The effect of different steel grade on the relevant behavior is captured according to the phase fractions and the properties of each phase. The model is validated using analytical solutions, and applied to explore the behavior of four different steel grades: Ultra-Low Carbon (0.003 %C), Low Carbon (0.04 %C), Peritectic (0.13 %C), and High Carbon (0.47 %C), simulating 30 s dwell times. Mesh refinement for capturing solidification details was examined and element sizes of 0.1 mm or smaller may be required to properly study solidification phenomena. All steel grades were found to follow the same general solidification behavior of compression at the surface increasing with time, and tension towards the solidification front. The initial solidification rate increases with carbon content. Thermal strain dominates the mechanical behavior. More stress and inelastic strain are generated in the high carbon steels, because they are mainly composed of highstrength austenite. Stress in the δ-ferrite phase is always very small, owing to the low strength of this phase. This simple model can help in the calculation of taper profiles for different steel grades and maintain the desired contact with the mold. While fixed linear taper practices are common in industry, this work demonstrates that the desired profile of the shell is actually parabolic. A thermal resistor model was developed to examine heat transfer phenomena within the interfacial gap during continuous casting. Results show that increasing slag layer thickness decreases heat flux across the gap. In addition, decreasing solidification temperature of the mold flux, for a fixed gap size, leads to more high-conductivity liquid present in the gap, so the heat flux rises. Finally, this work demonstrates an example of combining the realistic thermal-resistor model of the gap together with the thermal-mechanical model to show the different local behavior that can occur at a depression. Differences in flux layer thickness at the depression cause a drop in heat flux, higher surface temperatures, and a drop in shell thickness, which can result in necking phenomena in the solid shell as the stress from the shrinkage of the surrounding material concentrates in thinned shell regions. This work is the first step in demonstrating a transient 3-D model of thermal-mechanical behavior to examine the formation of cracks and depressions - where the correct behavior can be verified in a simplified case where an analytical solution exists, while allowing for extensions to higher dimensional modeling work in the future.
steel, finite-element model, numerical, heat transfer, stress, solidification, casting, phases
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign