This research investigates the impact of hot top characteristics on macroscale defects, specifically focusing on the severity and formation of macrosegregation in a 12MT mediumcarbon high-strength steel ingot. Macrosegregation poses significant challenges due to its resistance to elimination in subsequent processing stages, leading to non-homogeneities and potential product rejection, particularly in the case of large ingots. Critical measures involve the identification and control of macrosegregation sources such as alloy compositions, casting parameters, technique, and cast ingot setup configuration.
The hot top, situated at the upper part of the mold, comprises refractory sideboards on the sidewalls and a topping compound or board at the ingot's apex, playing a pivotal role within the cast ingot setup. Its functions include facilitating continuous feeding during casting, regulating heat flow at the ingot's apex during solidification, and providing a region for segregates and non-metallic inclusions to accumulate. Traditionally, hot top is cut to flatten the ingot's top surface and remove impurities. This process involves trimming off excess material from the hot top to achieve a smooth, clean surface on the ingot, thereby removing segregation and impurities from the top of the ingot.
In order to better understand the impact of hot top geometry on the extent and severity of macrosegregation, the casting and solidification process was simulated and experimentally validated. The commercial three-dimensional finite element modeling (FEM) code THERCAST® was used for the simulations. All the thermo-mechanical phenomena associated with mold filling, cooling in and out of the mold, and solidification based on a volume average two-phase model were simulated in the present study.
The model accounted for interactions between solute and heat transport coupled with flow driven by thermo-solutal convection and shrinkage-induced flow. Modeling boundary conditions were established based on industrial practices, incorporating considerations such as local friction, thermal exchange between steel and mold components, and mutual exchange among mold components within the casting system. Temperature-dependent parameters of all relevant materials were determined through combined approaches involving computational thermodynamics programs, materials software JMatPro® version 11.0, literature reports, THERCAST® software material database, industry sources, and theoretical calculations.
The casting setup for a 12 MT ingot, including the original industrial hot top configuration, underwent simulation, and the software and model's validity were confirmed through comprehensive experimental work on real industrial-size ingots. Experimental planning was meticulously devised based on key prediction results and casting parameters. To achieve this, a comprehensive set of tests was conducted. Chemical composition measurements were performed on the entire half-longitudinal section of the ingot to assess the segregation ratio of all elements. Macro etching was applied to the other half longitudinal section to reveal the macrosegregation pattern, while Dye Penetrant Inspection (DPI) was conducted on the half of the longitudinal section to detect microporosity. Additionally, monitoring of mold surface temperature and shrinkage cavity measurements were strategically carried out.
The investigation into the hot top configuration encompassed various aspects, including geometry, thermal conditions, and their combinations, utilizing the validated model for thorough analysis. The investigation focused on several variables within the hot top configuration, including hot top height, sideboard height, sideboard thermal conductivity, and preheating of the hot top, across eight various scenarios of the hot top configuration. These variables altered factors such as the mold mass ratio, mold slender ratio, and the thermal regime of the hot top, consequently affecting solidification kinetics.
Changes in the hot top configuration exerted significant influence on various aspects of the casting process, including the liquid metal velocity field, size, and shape of vortexes, cooling rate, liquidus temperature, temperature distribution, heat flux, solidification time, columnar to equiaxed transition position, and solidification profile. These factors collectively impacted the severity of macrosegregation within both the hot top and the ingot body. Additionally, microporosity and shrinkage cavity, two other critical casting defects, were investigated alongside macrosegregation to propose a solidification time criterion as a guideline for hot top configuration. A modified design was proposed to minimize macrosegregation, microporosity, and cavity formation.
This study provides deep insights into the underlying mechanisms responsible for macrosegregation occurrence in the ingot casting process. By elucidating the impact of the hot top on solidification kinetics, this research enhances understanding of the casting process design for specific ingots. These findings hold the potential for improving the quality of large ingot production and enhancing the productivity of high-value-added steels or other alloys prone to macrosegregation. Moreover, optimizing the use of hot top material and maximizing material efficiency can be achieved through the production of high-quality hot tops and ingot bodies.
| Date | 5 Sept 2024 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Mohammad Jahazi (Supervisor) |
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