6 June, 2012 Brüssel-Saal Ingot Casting – Simulation 2 ICRF 1 st International Conference on Ingot Casting, Rolling and Forging 1 Experimental and Numerical Studies on the Influence of Hot Top Conditions on Macrosegregation in an Industrial Steel Ingot Arvind Kumar, Miha Založnik † , Hervé Combeau Institut Jean Lamour, Ecole des Mines de Nancy, Parc de Saurupt – F- 54042 Nancy Cedex, France Joëlle Demurger, Jean Wendenbaum Ascometal Creas, Avenue de France, BP70045, 57301 Hagondange Cedex, France † Speaker contact: miha.zaloznik@ijl.nancy-universite.fr Abstract The influence of hot-topping conditions on macrosegregation in a large industrial steel ingot is experimentally and numerically studied. Two types of insulating refractory materials are considered in the hot top - isothermic (classical hot topping) and exothermic. Unlike the isothermic material, the exothermic material releases heat into the solidifying ingot by an exothermic chemical reaction taking place during a certain period. Experimental macrosegregation and grain size at the central axis in a 6.2-ton industrial steel ingot are presented for the two hot top conditions. Thereafter, numerical studies using a multiscale model and confrontation between the experimental and numerical results are presented. In the multiscale model, the grain growth model is fully coupled with a volume-averaged two-phase macroscopic solidification model that takes into account macroscopic fluid flow, grain transport, heat transfer, and solute transport. The effects of the two hot top scenarios on the heat transfer behaviour, macrostructure and the macrosegregation are discussed. We have found that the insulating refractory material in the hot top plays an important role in the grain formation with exothermic refractory material resulting in a finer axial grain structure with more globular grain morphology and more macrosegregation. The model results well explain the experimental results for the two hot top scenarios. The exothermic chemical reaction can increase the number of nuclei, which in turn influences the grain structure and the macrosegregation in the ingot. Key Words: Large steel ingot, Hot top, Isothermic, Exothermic, Macrosegregation, Multiscale solidification model. 1. Introduction The production of steel ingots with improved structure and chemical homogeneities is of great concern for steelmakers. Prediction of grain structure and chemical heterogeneities in ingots (especially in industrial-size products) using numerical models is of great importance as it can significantly improve the production efficiency. The development of solidification models for industrial steel ingot processes is a challenging task mainly due to the size of the products and the need to consider variety of physical phenomena [1-4]. Some of those various phenomena need to be considered are thermosolutal convection [5,6], grain motion [1-4,7-9], evolution of grain morphology by suitably considering a coupled grain growth model in the macroscopic solidification model [10-12], formation of channel segregates [5,6]. Chemical heterogeneities and grain structures significantly influence the quality and final properties in solidified ingots. Moreover, the phenomena responsible for their formation during solidification are closely related [1-6]. However, the development of models combining these two coupled aspects is still at its beginning. Vannier et al. [5] reported calculations for a large multicomponent steel ingot, however, grain motion and evolution of grain morphology during solidification was not considered which present limitations of their study. In the previous works Combeau and coworkers [1,3] developed a multiscale model considering motion of equiaxed grains and evolution of grain morphology during solidification. In the multiscale model the grain growth model is fully coupled with a volume-averaged two-phase macroscopic solidification model that takes into account macroscopic fluid flow, grain transport, heat transfer, and solute transport. Simulation results for a 3.3-ton steel ingot are reported considering the motion of equiaxed grains [1] but not the grain morphology evolution. In practice, the grain morphology in large ingots experiences transitions [2,11,12]; we generally find globular regions at the bottom and dendritic regions in the central part of the ingot. Therefore, evolution of the grain morphology has to be accounted for in the solidification model in order to properly predict the macrosegregation [2]. With this aim, Combeau and coworkers considered the evolution of grain morphology during solidification and presented the results for a 3.3-ton steel ingot [11, 12]. Very recently, equiaxed grain structure and macrosegregation in even a larger 6.2 ton industrial steel ingot were predicted [4]. The experimental and predicted macrosegregation pattern showed good agreement [4]. While we know that free-floating grains appear during the solidification of an ingot [1-4,9], the cooperation and competition of the grain-settling effect with that of the melt flow depends on the morphology of the grains