A microsegregation model in the two-phase region of an ND steel continuous casting billet

ND steel is a low alloy steel that resists the dew point corrosion of sulfuric acid.To improve the special performance of ND steel, the chemical composition of ND steel not only contains conventional elements but also adds corrosion-resistant elements, such as Cu, Cr, and Ni.During the solidification process, the molten steel will undergo a phase change reaction.Owing to the differences in the distribution coefficients and diffusion coefficients of solute elements in different phases, solute elements will be redistributed in the solid-liquid two-phase region during solidification, which will lead to microsegregation of solute elements.The microsegregation of solute element makes the zero strength temperature and zero plasticity temperature (ZDT) of steel decrease, which makes the temperature range of brittleness expand and deteriorates the mechanical property of high temperature of the continuous casting billet, and finally increases the probability of inducing surface cracks.This paper takes the microsegregation of solute elements as the research background.Herein, a microsegregation model for the solute in the solidified two-phase region of an ND steel continuous casting billet was established.In the model, the effects of elements C, S, and P on high-temperature mechanical parameters and solute redistribution of steel in its solid-liquid two-phase region were studied, and the variation law of the segregation ratio of elemental P with cooling rate (C_R) was also explored.According to the analysis of the model results, when the initial C content was between 0.075%and 0.125%, with an increase in the initial C content, segregation of P and S elements intensified, and the temperature drop at the solidification end became larger, leading to the increase in the brittle temperature range.According to the analysis of the model results, increasing the initial content of P and S will decrease the segregation ratio of P and S elements but will increase the enrichment content of P and S elements in the residual liquid phase between dendrites, directly leading to the decline of ZDT.Analysis of the model results shows that the Cu content in ND steel is lower than the critical content that significantly increases the crack sensitivity, and the segregation ratio of Cu element is at a low level during solidification.Therefore, elemental Cu cannot dominate the induced crack in ND steel during solidification.Finally, within a certain range of cooling rate fluctuation, the segregation ratio of P will decrease slightly with increasing C_R.