Research On Solidification Process And Solidification Structure Of Silicon Steel Strip

Thin strip continuous casting is an advanced technology in the fields of ferrous metallurgy and materials processing, that permits the production of thin strip directly from the liquid steel and integrates casting and rolling. Silicon steels are widely used in electronic and electric power applications because of their excellent magnetic properties, which are deemed to one of the most promising steel in twin roll casting. Research on the solidification behavior of silicon steel strip has important theoretical and actual value. This thesis investigated the solidification process of silicon steel strip, and conducted an exploratory research on the application of electric current pulse (ECP) onto thin strip continuous casting process.An experimental investigation was carried out on a vacuum non consumable arc-melting innovated to approximate the near rapid solidification conditions of thin strip continuous casting. The influence of alloy composition and cooling rate on solidification structure was analyzed by suction casting experiments. The results show that the rate of equiaxed grain zone of silicon strip was increased with decreasing cooling rate. But the rate of equiaxed grain zone of the silicon steel strip with Si content of showed the oppositional result. The reason for the oppositional difference is considered that the linear shrinkage of the silicon steel strip with 4.72wt. % Si is higher than the other Si content strips. The contraction during solidification led to the reduction of heat flux, because of the formation of gaps, which could increase the heat resistance. In solidification structure of silicon steel strip with 2mm thickness, the increase of Mn content had little effect on the proportion of equiaxed zone. In solidification structure of silicon steel strip with 4mm thickness, the rate of equiaxed zone decreased with the increasing of Mn content until the content reach to 0.76wt. %, then the rate was increased. In solidification structure of silicon steel strip with 6mm thickness, the rate of equiaxed zone decreased with the increasing of the Mn content. The concentration distribution was investigated in the silicon steel strips, and the results showed that the homogeneity improved with higher cooling rate, and the distribution of silicon elements was affected by the dendrite structure evolution.The solidification structure evolutions of silicon steel under the near rapid solidification conditions was analyzed foundation of dendrite growth theories (LKT model). The results showed that the curvature undercooling firstly increased with the increasing of growth velocity, then decreased when the velocity up to 0.06m/s. The interfacial kinetic undercooling and solutal undercooling increased with the increasing of the velocity, and the values were very small. The dendrite tip radius and primary dendrite arm spacing decreased with the velocity. The shape of liquid-solid interface was changed from dendrites to cells, when the velocity exceeded 0.03624m/s. The shape of liquid-solid interface was changed to planar front growth when the velocity reached 0.125m/s. Growth map of isotherm velocity against temperature gradient for silicon steel was presented based on the KGT model and the CET model.The precipitation of inclusions analysis showed that the inclusions were almost oxides, which would be diffuse distribution in a local region, or linear distribution, or chain distribution, or compound congregated. With the same silicon content, the diameter of the biggest inclusion (DBI) decreased with the increasing cooling rate of strips. With the same cooling condition, the DBI decreased with the increasing Silicon content.Copper wedge mould was used to simulate the molten pool formation condition of actual thin strip continuous casting, and electric current pulse (ECP) was applied on silicon steel solidification process to investigate the influence of ECP on silicon steel solidification structure, especially the rate of equiaxed zone. It can be found that the solidification structure of silicon steel without ECP treated was almost full columnar grain, on the contrary, equiaxed grain zone was increased and columnar grain zone was decreased remarkably by exerting ECP. Increasing of peak value current and pulse discharging frequency could increase the rate of equiaxed zone, but the trend of the increasing rate would become slower when they reach the value of 10K_i A, 20H_i Hz respectively. The effect of electrodes inserted modes and the discharging time on the solidification structure was also discussed. The results showed that the action zone of ECP focused on the top surface and region near the copper substrates, which was determined by measurement of dendrite arm spacing. It is considered that the floating crystal nuclei caused by ECP in melt which would drift into the liquid, were generated from the top surface and solidified shell near the copper substrates. The purpose of the thesis is to evaluate the effects of ECP on the increase of equiaxed zone of silicon steel in a wedge copper mould, and thus to analyze the possibility of exerting ECP on the continuous casting process. The experimental results indicates that ECP could be employed onto thin strip continuous casting process to improve the rate of equiaxed zone of strip, and the increase of equiaxed zone is deemed a feasible approach to obviate the corrugated defect of the silicon steel strip in the following roll stage.