Research on internal crack susceptibility of continuous-casting bloom based on micro-segregation model

The solidification and cooling of liquid steel in continuous casting process is a complicated nonequilibrium phenomenon. During steel solidification process, the micro-segregation of solute elements between liquid steel and solidified shell will vary with their temperature-dependent diffusion coefficients and equilibrium distribution coefficients. Due to non-uniform cooling pattern in the continuous casting process of steel blooms, the fluctuation of cooling rate in bloom will have a great influence on micro-segregation degree of the elements. The micro-segregation behavior of solute elements in steel solidification process is responsible for the variation of characteristic temperatures such as zero strength temperature (ZST), zero ductility temperature (ZDT) and liquid impenetrable temperature (LIT), which make up the brittle temperature range in steel solidification. During continuous casting process of steel, internal cracks created by thermal and mechanical deformation tend to occur in this range. To prevent the occurrence of these cracks in continuous casting bloom, it is essential to better understand about the internal crack susceptibility concerning micro-segregation behavior in the non-uniform cooling process. In this work, a micro-segregation analytical model for YQ450NQR1 steel continuous casting bloom is established to study the inter-dendritic segregation behavior of main solute elements C, Si, Mn, P and S at various cooling rates, the results show that P and S are more likely to segregate compared with C, Si and Mn and the increase of cooling rate weakens the micro-segregation degree of C, Si, Mn, P and S. Based on the micro-segregation model established above, ZST, ZDT and LIT for YQ450NQR1 steel are calculated and the influences of cooling rate on ZST, ZDT and LIT are analyzed. It reveals that ZST, ZDT and LIT of YQ450NQR1 steel bloom decrease accordingly with the increase of cooling rate. On this basis, the index of internal crack susceptibility (IICS) is defined to quantitatively characterize the internal crack susceptibility of the bloom. The results show that the internal crack susceptibility becomes larger while the IICS value approaches to 1. Furthermore, an internal crack susceptibility model is obtained concerning IICS and cooling rate (C_R) and the validation is performed to certify the model's suitability in quantitatively predicting internal crack susceptibility of YQ450NQR1 steel continuous casting bloom in the non-uniform cooling process.