Simulation Of Microstructure For Steel Solidification During Slab Continuous Casting Process

The quality of the final steel products is the most concerned issue in the industrial practice of steel continuous casting process. It is very important to investigate the dendrite evolution and microsegregation among the dendrites for steel solidification during continuous casting process. The mathematical simulation has become a necessary technology in the micro field. Nowadays researches on the microstructure evolution and the micro-segregation in steel continuous casting process have received more and more interests. Dendrite is one of the most common configurations during solidification. For continuous casting process, the analysis of the dendrite growth and the microstructure shape is quite different from that of ingot casting process. In the present study, the microstructure and microsegregation of steel solidification combined with the macro- heat transfer have been studied for continuous casting process. The main contents and the developments are carried out below:(1) The microstructure and microsegregation models which are summarized both for the continuous casting process and the ingot casting process are presented for the application of steel solidification during continuous casting process. A numerical method is developed to analyze and calculate the microstructure parameters, such as the dendrite tip radius, the dendrite growth velocity, the liquid concentration, the temperature gradient, the cooling rate, the secondary dendrite arm spacing, and the dendrite tip temperature in front of S/L interface, with the variations of solidification progress and solid shell growth for both carbon steel and stainless steel during continuous casting.(2) The current model is well validated by the published models and the measurement data. The results show that the present model is reasonable. Taking carbon steel for actual continuous casting, the analysis of secondary dendrite arm spacing of the samples is compared with the calculation results. The reasonable stability coefficient in the range of 0.05-0.085 and Gibbs-Thomson coefficient 1.2×10^-8 are determined for continuous casting process.(3) The microsegregation of C,Si,Mn,P,S for carbon steel and C,Cr,Ni,P,S for stainless steel with the variations of both solidification progress and solid shell growth are discussed. When the solid fraction is over 0.9, the microsegregation of P and S increases observably, especially the S segregation, while the variation of C, Si and Mn (Cr and Ni) microsegregation is not so obvious. With the solidification progress, the microsegregation degree of each element increases with the increasing of the dendrite growth velocity.(4) The effects of the stability coefficientσ^* and casting speed on the microstructure parameters are studied. For different systems, a* takes different values. The determination of the stability coefficient value has great importance on the dendrite growth shapes forming. The larger the stability coefficient is, the smaller the dendrite tip radius and secondary dendrite arm spacing are. When the stability coefficient changes from 0.025 to 0.05, the dendrite tip radius decreases by 5-6μm for both carbon steel and stainless steel. But the reduced rate for the secondary dendrite arm spacing for carbon steel is within 100μm, which for the stainless steel is only within 10μm. Effects of different casting speed V_c 0.6m/min, 0.8m/min and 1.0m/min on these parameters are also investigated. When casting speed increases from 0.6 m/min to 1.0 m/min, the dendrite growth velocity, the solute concentration, the temperature gradient, the cooling rate and the undercooling decrease, while the dendrite tip radius, the secondary dendrite arm spacing and the dendrite tip temperature increase.(5) Comparisons between the carbon steel and the stainless steel for the above results are carried out, and the same trend for both two steel grades is obtained. When the dendrite growth rate changes from 0.1 mm/s to 0.7 mm/s, the dendrite tip radius for carbon steel changes within 12μm, while dendrite tip radius for stainless steel is 3-4μm smaller than that of carbon steel. For the secondary dendrite arm spacing, the values of stainless steel are much smaller than those of carbon steels. The comparisons show that the micro- structural parameter values of stainless steel are smaller than those of the carbon steel due to its special compositions and solidification mode characteristics.Above all, the current study is just an exploration and has obtained some elementary developments. More research should be done to realize the prediction of the microstructure shape and improve the capabilities of microstructure for the solidification process during continuous casting.