
Abstract
The influence of hot top designs with different heat capacities on the distribution of positive and negative macrosegregation was investigated on a 12 metric-ton cast ingot made of high-strength steel. The three-dimensional finite element modeling code THERCAST® was used to simulate the thermo-mechanical phenomena associated with the solidification process. The model was validated on an industrial-scale ingot and then used to evaluate the influence of the hot top's thermal history, a crucial component in a cast ingot setup. This evaluation aimed to comprehend changes in solidification time, temperature and heat flux, all of which contribute to determining the severity of macrosegregation. Preheating the hot top had minimal impact on solidification time, but increasing thermal conductivity extended solidification by 31%, significantly reducing positive carbon segregation to 13% in the ingot body and 23% in the hot top.
Keywords: hot top; finite element modeling; ingot casting; macrosegregation; heat capacity
Using a Hot-Top to Mitigate Macrosegregation
Ingot casting followed by forging is the main method for producing large specialty steel components, such as mill rolls and turbine rotors. These steels typically contain up to 10 alloying elements to ensure the desired mechanical strength and corrosion resistance. During solidification, heat extraction through the mold, combined with differing diffusion rates of the alloying elements, results in a heterogeneous distribution of the alloying elements—over distances ranging from centimeters to meters—called macrosegregation. This negatively affects the mechanical properties of the final products, and often, particularly in the case of large ingots, cannot be eliminated even after extending homogenization heat treatment.
A hot top, Fig1, located at the top of the mold, significantly influences the solidification process and macrosegregation sensitivity by regulating heat flow and continuously supplying liquid metal as the ingot solidifies. Made of cast iron and lined with refractory bricks, it features exothermic caps that generate heat upon contact with molten metal. Therefore, managing the heat exchange and thermal regime of the hot top is essential for reducing solidification defects like macrosegregation.

The aim of this study was to quantify the influence of a hot top thermal regime on macrosegregation severity and solidification kinetics in a 12 MT ingot. Four heat capacity scenarios were analyzed using a validated finite element model (FEM) based on industrial experiments.
Materials and Methods
A medium-carbon low-alloy steel was bottom-poured into a 12 MT mold after processing in an electric arc furnace, ladle furnace treatment, and vacuum degassing. A 25.4 mm thick longitudinal slice was taken from the ingot's center for chemical mapping (370 specimens) and macro-etching (10 plates). Chemical analysis was conducted using an optical emission spectrometer, and macrosegregation patterns were mapped using a developed MATLAB® code.
Macrosegregation was modeled using the THERCAST® finite element code, with a 3D simulation of the solidification process considering fluid flow, temperature, and solute distribution. An Arbitrary Lagrangian-Eulerian (ALE) formulation, based on a volume-average two-phase model, described the thermo-mechanical phenomena during mold filling and cooling. Heat transfer was analyzed for various subdomains (mold, hot top, ingot) under boundary conditions such as convection, radiation, and imposed heat flux. A Fourier-type equation was used to evaluate the contact resistance at the metal-mold interfaces. The model was validated against experimental data. Figures 2a and b illustrate the sample preparation approach and a 180° model of the 12 MT ingot.


Hot Top Designs
Four new hot top designs (ND1 to ND4) were compared to the original design (OD). The latter features a refractory sideboard with a thermal conductivity of 1.23 W/m/K. ND1 considers preheating the hot top to 200 °C; ND2 reduces the sideboard’s thermal conductivity to 0.45 W/m/K, and ND3 and ND4 add an extra 177 mm refractory layer. In ND4, the upper refractory thermal conductivity is reduced to 0.45 W/m/K. Figure 3 illustrates the designs.

Solidification Kinetics and Macrosegregation
It is critical to understand how changes in hot top designs affect solidification times. Total solidification time is the time needed for the metal to cool from its initial temperature to the solidus temperature, i.e., the temperature at which all the metal has solidified. Local solidification time refers to the period between liquidus and solidus temperatures, i.e., time in the mushy zone. Figure 4 shows that variations in the thermal regime of the hot top significantly impact both total and local solidification times, as revealed by simulation results.



Figure 5 shows the carbon segregation ratio for all designs, arranged by increasing total solidification time. The maximum segregation ratio decreases in the bottom, middle, top, hot top, and centerline regions as a function of the hot top thermal regime. This reduction is likely due to higher local temperatures, resulting in longer solidification times and increased diffusion times for solute elements in the new designs compared to the original one (OD).



This study allowed us to determine that by changing the hot top thermal regime, it was possible to modify the solidification kinetics (between 2% to 31% increase in solidification time). This also allowed us to reduce the positive carbon macrosegregation by 13% in the body and by 23% in the hot top region. Longer solidification times improved solute diffusion, reducing carbon macrosegregation as the most important segregation element affecting the mechanical properties of the cast ingot.
Additional Information
For more information on this research, please read the following papers:
Ghodrati, N.; Champliaud, H.; Morin, J.-B.; Jahazi, M. Influence of the Hot-Top Thermal Regime on the Severity and Extent of Macrosegregation in Large-Size Steel Ingots. J. Manuf. Mater. Process. 2024, 8, 74. https://doi.org/10.3390/jmmp8020074
Ghodrati, N.; Baiteche, M.; Loucif, A.; Gallego, P.I.; Jean-Benoit, M.; Jahazi, M. Influence of Hot Top Height on Macrosegregation and Material Yield in a Large-Size Cast Steel Ingot Using Modeling and Experimental Validation. Metals 2022, 12, 1906.
https://doi.org/10.3390/met12111906
Ghodrati, N., et al. (2023). Influence of Hot Top Geometry on the Solidification Time and Macrosegregation in Large-Size Cast Ingot Using Finite Element Modeling. the 62nd Conference of Metallurgists, COM 2023, Toronto, Ontario, Canada, Springer, Cham, p. 477-480. https://doi.org/10.1007/978-3-031-38141-6_65. ,
Ghodrati, N., et al. (2022). Modeling of the influence of hot top design on microporosity and shrinkage cavity in large-size cast steel ingots. 8th International Congress on the Science and Technology of Steelmaking, Montreal, QC, Canada, p. 239-244.