Influence of solidified structure on center segregation of saw blade steel 75Cr1 in CSP process

To address the center segregation issue of 75Cr1 high-carbon low-alloy tool steel produced through the domestic CSP process without electromagnetic stirring (EMS), this study systematically investigates the inhibition mechanism of the CSP process on center segregation by integrating multiscale and micro-characterization with thermodynamic calculations, fractal dimension analysis, and other methods. The results show that: slab bulging in the CSP process significantly enhances the driving force of molten steel flow and exacerbates the macrosegregation at the slab center; after process optimization, slab bulging is mitigated, leading to a substantial reduction in center macrosegregation. Meanwhile, columnar dendrites penetrate the center to form a transcrystalline structure with refined grains, where the secondary dendrite arm spacing ranges from 21 to 183 μm and the permeability from the slab surface to the center is 0.1 to 5.1 μm 2 . The cooling rate of the CSP process is nearly 15 times higher than that of the traditional process, ranging from 0.5 to 25 K/s. Electron Probe Microanalysis (EPMA) results demonstrate that C, Mn and Cr are enriched toward the center, C, Mn and Cr show similar segregation trends, while Si is uniformly distributed in the form of discrete spots. Thermodynamic calculations indicate that solute redistribution is concentrated at the solid-liquid interface; the equilibrium distribution coefficient of C is small and decreases with decreasing temperature, rendering it prone to segregation; the equilibrium distribution coefficients of Cr and Mn deviate further from 1 as temperature drops, resulting in intensified segregation; whereas that of Si approaches 1, leading to a low segregation tendency, which suggests that the CSP process exerts distinct segregation inhibition mechanisms for different elements. Calculations of fractal dimension and generalized entropy reveal that the self-similarity of the structure decreases from the slab edge to the center line, the negative entropy flow increases, the system entropy decreases, and the disorder degree of the solidification system is reduced. The findings of this study provide theoretical and empirical support for understanding the regulation mechanism of the CSP process on the center segregation of 75Cr1 steel.