Macro- and microstructural evolution, compositional segregation, and property changes in 45Si2MnCr2Mo steel produced by CSFC with a tensile strength of 2.35 GPa compared to the TC process: CFD-based temperature gradient simulation and experimental characterization

Two 45Si2MnCr2Mo ultra-high strength steel (UHSS) ingots were prepared using traditional casting (TC) and composite shear flow casting (CSFC), respectively. Initially, a CFD-based simulation was performed to analyze the temperature field during solidification, revealing that the CSFC process can significantly reduce the temperature gradient compared to conventional casting. Based on these simulation results, the CSFC process was applied to fabricate ingots. The characteristics of the CSFC ingots were then analyzed in terms of chemical composition and macro-and microstructure. The results showed that the reduced temperature gradient effectively suppressed composition segregation, inhibited the formation of columnar dendrites, and promoted the formation and refinement of equiaxed grains and improved sub-structural characteristics. The proportion of equiaxed zone increased from 4.4 vol% in TC to 55.4 vol% in CSFC, while the columnar zone decreased from 72.25 vol% to 20.28 vol%, with the chill zone remaining nearly unchanged (∼24 vol%). These experimental observations of the three-zone distribution are consistent with the predictions from the CFD-based simulation, confirming the effectiveness of the CSFC process in controlling the solidification structure. The grains with low-angle grain boundaries inside the CSFC ingot are as high as 25 %, and contain a large number of small clusters with similar orientations. After identical rolling and heat treatment, the orientation clusters in the CSFC ingot rotated during deformation, promoting fine grain formation and thereby refining the lath martensite structure. It was found that the CSFC-treated samples exhibited a remarkable tensile strength of up to 2352 MPa, with increases of 240 MPa in tensile strength and 107 MPa in yield strength compared to conventional casting. Meanwhile, the total elongation improved significantly from 7.7 % to 12.1 %.