Formation and Control of Large Oxide Inclusions at the Spot-Segregation Defects of Al-Killed GCr15SiMn Ingots

The form process of large oxide inclusions at the spot-segregation defects of Al-killed GCr15SiMn ingots and their influence on segregation defects were studied by conducting industrial sampling and thermodynamic analyses. Specifically, by comparing the distributions and numbers of three-dimensional inclusions in the segregation defect and normal areas in the hot-forged bar by X-ray microtomography, we found that in addition to the enrichment of light solute elements, an increase of the number of large spindle-shaped inclusions in the mushy zone aggravates formation of segregation defects. The average composition of the large inclusions was 90 mass pct (CaO + Al2O3)-5 mass pct SiO2-5 mass pact MgO. The characteristics and formation process of oxide inclusions in the refining process were then investigated. The results showed that the large inclusions containing SiO2 originated from the endogenous liquid inclusions generated during the vacuum-degassing process. The main reason for formation of large inclusions with size greater than 5 mu m was that the proportion of CaO in the inclusions exceeded 41.5 mass pct. The liquid inclusions showed obvious heritability and easily remained in the ingots, affecting the solidification process and leading to segregation defects. Finally, the optimization process of adjusting the aluminum content and slag composition in the ladle-furnace refining process of GCr15SiMn steel was proposed and verified by industrial tests. It was found that spot-segregation defects of the ingots were suppressed by effectively reducing the number of large CaO-Al2O3-SiO2-MgO inclusions.