Undesirable macro segregation defects, freckles, restrict the commercial production of large-sized electroslag remelting (ESR) bearing steel ingots through degradation of the mechanical properties and service lifetime. In order to clarify the freckle characteristics and formation tendency as well as the formation mechanism, freckles from an industrial large-sized GCr15SiMn ESR ingot were investigated through structural and compositional analysis, along with simulation calculation. The results show that freckles consist of (Si, Mn, Cr)-enriched equiaxed grains and occur in about the 1/2 radius region at the middle-upper part of the ESR ingot, where the secondary dendritic arm spacing (SDAS) and solidification front angle are large but cooling rate is small. The absolute value of relative Rayleigh number, Ra, also reaches its maximum in the 1/2 radius region, with a liquid fraction of 0.3–0.5, corresponding to the region where freckles form. Based on the experimental and simulation results, to evaluate the freckle formation in industrial-scale GCr15SiMn ESR ingots, the threshold value of relative Ra, a freckle criterion considering the compositional and thermal effects, was determined to be about −0.023.

In this study, a new method is proposed for the plasticization of inclusions by taking advantage of the behavior of alkali metals in lowering the inclusion melting point. A series of experiments with NaF additions to molten steel were carried out using a carbon tube furnace followed by simulated rolling tests using the solidified ingots and characterization of inclusions with the help of automated scanning electron microscopy. Compositional changes of the steel and evolution of gaseous species were evaluated using thermodynamic software FactSage 7.2 (ThermFact Inc., Montreal, QC, Canada). Based on this approach, NaF/steel/inclusion interactions and the effects of NaF addition on the melting point, size and deformability of inclusions were clarified. The modification of MnO-SiO2 inclusions by NaF also promoted the removal of inclusions and improved the cleanliness of steel. The results show that with the addition of NaF, the melting point of inclusions is greatly reduced, the deformability is improved, and the removal of inclusions is enhanced, all of which indicates a good prospect for industrial application.