Numerical Simulation And Parameter Description On Solidification Structure Of Twin-roll Thin Strips

Twin-roll thin strip casting is thought as a potential technology in the field of metallurgy and materials. It has been found that the main problem of its industrialization application is that the quality of thin strip is poor and unstable. Since the solidification structure of thin strip has great effect on the strip quality, it is very necessary to investigate the influence of the processing parameters on the solidification structure of thin strips. Considering that the relationship among these processing parameters is very complicated, the experimental investigation would be very hard and difficult. In order to solve this problem, the numerical simulation is used in this paper, by which the research work can be obviously simplified. Because the processing performance and final properties of thin strip are affected by the distribution feature of the crystal zones and the three-dimensional size of the crystal grains in different crystal zones, it is also very important to carry out the investigation on parameter description of the solidification structure of twin-roll thin strips. A general mathematical model which connects macroscopic heat and mass transfer with microscopic nucleation and grain growth during the solidification process of twin-roll thin strip was firstly developed in this paper. In this model, the latent heat release was treated by the enthalpy method and reseau and node were divided by streamline boundary. The heterogeneous nucleation model and the competition growth mechanism between columnar dendrite and equiaxed dendrite were introduced, together with the revising of the dynamic model, KGT model, which was originally established under the pure diffusion condition to simulate the dendrite tip growth during the rapid solidification process. Since the solid fraction is expressed as the function of nucleation density and growth velocity and is not treated as the function of temperature in this model, the coupling between solidification thermodynamics and crystallization kinetics during the solidification process of twin-roll thin strip can be realized truly. As a result, the problem that only temperature field was simulated and the solidification structure feature could not be predicted by early mathematical models can be solved. With the help of solid fraction, the coupling of the macro/micro aspects was realized by using two different time-steps, macro and micro, together with the tracing technique of columnar dendrite front. It was found that the calculated results by this model were in agreement with the experimental values. Therefore, the model can be used to predict the effects of the processing parameters on the solidification structure of twin-roll thin strips. On the basis of the mathematical model developed in this paper, the effects of the pouring temperature, casting velocity and pool height on the proportion of columnar crystal zone, the columnar dendrite spacing and the deflection angle were first quantitatively clarified by numerical simulation, which provides the theoretical basis for both the optimization of the processing parameters and the control of the solidification structure and strip quality. A set of description system of the solidification structure parameters of twin-roll thin strip was also firstly established in the paper according to the quantitative metallography and probability theory, which provided an effective method for the quantitative description of the solidification structure feature of twin-roll thin strip. According to the description system established in this paper, the quantitative description of both the distribution feature of the crystal zones and the three-dimensional size of the crystal grains in different crystal zones can be realized by just selecting three specific metallographic milling surfaces with different orientations, which greatly simplified the experiment work. Comparing with the traditional description method, the description system established in this paper can truly reveal the three-dimensional size of the cry...