Integrated Casting-Forging Modeling and Back-Tracking Analysis of Macrosegregation for Extreme Hot Top Design in Ultra-large Steel Forgings

To achieve effective control of macrosegregation in large steel ingots and maximize material utilization, an integrated casting–forging modeling framework for the formation and back-tracking of metallurgical defects was developed. Based on this approach, the formation and evolution of macrosegregation under different process conditions were systematically investigated. The results show that increasing the hot top height from 250 to 950 mm reduces the segregation exceedance radius at the hot top line from 0.604 to 0.399 m, indicating that macrosegregation defects become increasingly concentrated within the hot top region. Increasing the hot top height significantly reduces solute enrichment in the outer usable region of the ingot, while the segregation level in the central region exhibits a non-monotonic variation with a peak at intermediate hot top heights. For hollow forgings where the center region is removed during piercing, a larger hot top height effectively reduces segregation along the inner wall of the forging. The influence of hot top insulation is strongly dependent on the hot top height and mainly leads to an upward redistribution of macrosegregation within the ingot. Based on the proposed model and defect back-tracking analysis, an extreme hot top height of 300 mm was determined for the 225 t ingot used to produce an ultra-large hollow cylindrical forging. Compared with the conventional 650 mm hot top, the proposed design saves approximately 19 t of molten steel per casting cycle and approximately 13.0 MWh of electricity per cycle in electric-furnace melting and refining. Industrial production trials confirmed the quality of the resulting forging. The proposed modeling and process design methodology provides an effective tool for optimizing casting–forging processes of large steel forgings via extreme hot top design.