Influence of Hot Top Height on Macrosegregation and Material Yield in a Large-Size Cast Steel Ingot Using Modeling and Experimental Validation

Abstract

The effect of the hot top height on the formation of positive and negative macrosegregation patterns, the ingot quality, and the material yield during solidification of a 12 MT cast ingot made of a Cr-Mo-low alloy steel was investigated. A 3D numerical simulation of the process was conducted using finite element modeling. A full-size 12 MT ingot was cut off from its center in the longitudinal direction, and a large cross-section was sliced into small samples. The chemical mapping of all the elements in the steel composition was obtained for all samples and compared with the model predictions for validation purposes. The influence of the increase in hot top height on the liquid metal velocity field, size and shape of vortexes, cooling rate of the liquid, and liquidus temperature was determined. Results revealed that increasing the hot top height by 165 mm increased the solidification time, fluid velocity in regions including the hot top and ingot bottom, and decreased the local liquidus temperature. The combination of all the above resulted in an overall decrease in positive and negative macrosegregation of more than 6% and an increase in ingot quality. The results are interpreted based on the interactions between the transport of solute and heat coupled with the flow driven by thermo-solutal convection and shrinkage-induced flow.

Influence of the Hot-Top Thermal Regime on the Severity and Extent of Macrosegregation in Large-Size Steel Ingots

“…Then, macrosegregation patterns of elements on the total part of the longitudinal section were built using MATLAB ® version 2012 (The MathWorks Inc., Natick, MA, USA) [22]. The surfaces of the other 10 plates were etched using a 50% HCl-50% H 2 O solution at 50 • C to reveal the patterns of macrosegregation [4]. This experimental study was conducted to gain insights into the intensity and evolution of segregation for all alloying elements.…”

“…Laboratory-scale experiments are often not representative of the industrial reality, and empirical industrial-scale testing is extremely costly and time-consuming. Therefore, numerical approaches are needed to address these intricate physical phenomena that arise throughout the casting process, allowing for a faster optimization of the casting conditions as well as the quantification of the interactions between and mutual influences of the different parameters [3][4][5][6][7][8][9][10].…”

“…Among the various components of the cast ingot setup, the hot-top is identified as the most important one [4]. However, other components such as the mold, refractory material, runner, and trumpet also impact the solidification of the ingot.…”

“…The hot-top plays a crucial role in the solidification process and consequently influences the macrosegregation sensitivity of the ingot because it provides the heat flow at the top of the ingot and continuously feeds the ingot with liquid metal as it solidifies. Thus, controlling the heat exchange and thermal regime of the hot-top is of critical importance in decreasing solidification defects like macrosegregation [4,13,14].…”

“…The case study is a 12 MT bottom-poured ingot of high-strength steel used in energy and transportation systems that require very high levels of microstructure homogeneity. The developed FEM model was first experimentally validated on an industrial-size ingot [4] and then used to evaluate the impact of the four investigated thermal regime scenarios.…”