Contribution Oral Presentation
MACROSEGREGATION FORMATION AND CONTROL IN BEARING STEEL VIA NUMERICAL SIMULATION AND EXPERIMENTAL CHARACTERIZATION
- Prof. Dianzhong LI
- Prof. Dianzhong LI (Institute of Metal Research,Chinese Academy of Sciences)
Chemical inhomogeneity and inclusions are the most severe defect types in bearing steel which have great influences on the ingot quality and final product properties. Based on a series of full-dissections of steel ingots varying with composition and weight, numerous experimental characterizations by SEM, EPMA, TEM, ASPEX and 3D Micro-CT techniques and multi-scale simulations, a new driving force, inclusion flotation, has been discovered to drive the formation of solutal macrosegregation, especially channel segregation. Sufficient populations of light oxide-based inclusions with appropriate size are able to alter the local flow pattern and destabilize the mushy zone, consequently dominating the channel segregation formation. Accordingly, a significant concept to control the macrosegregation in practice is the low-oxygen purifying treatment of the melt and inclusion modification, such as via Rare earth addition. Actually, even with a highly low proportional of RE’s additions in industrial-level 100Cr6 bearing steel ingots, inclusions can be effectively modified into smaller and much more compatible RE-O-S type with Fe matrix, and over 60% large-size inclusions beyond 5 μm disappear, avoiding the growth of large-size MnS/Al2O3. In addition, the small density contrast between RE-O-S inclusions and steels also leads to a slow flowing among inter-dendritic regions. These two factors reduce and even eliminate channel segregation, and also lower the global macrosegregation extent significantly. The magic of RE treatment on segregation control has been widely verified and applied in other special steels of mould, gear and nuclear power component. Besides, the appropriate casting process and optimized ingot design are also significant factors to reduce macrosegregation in bearing steel. For instance, the higher superheat degree above 40 oC induces the longer solidification time and stronger interdendritic natural convection; meanwhile the decrease of the local cooling rate coarsens the dendrite arm, consequently elevating the mushy zone permeability. Obviously, both the large density difference and coarse microstructure provoke the severity of macrosegregation. On the other hand, the larger height, narrower shoulder width and the smaller taper in the riser are beneficial to shifting the hot spot and the maximum segregation value from the ingot body towards the riser top. In summary, the current study highlights the importance to produce super-homogenized bearing steel by jointly purifying the steel melt, modifying the inclusions, lowering superheat degree and ideal hot-top design.