Research Of Numerical Simulation For Thin Slab Continuous Casting With Liquid Core Reduction

The technology of thin slab continuous casting and rolling is one of the most important revolutionary technologies after the oxygen converter steelmaking and the continuous casting of steel in iron and steel industry. Since our country introduced the 1st production line at the end of last century, thin slab continuous casting and rolling has been developed at a full speed, and the various research works launched in succession too. Compared with traditional production technology, the new technology belongs to one compact type short procedure and near net shape continuous casting (NNSCC). It can save investment, lower costs, improve the slab quality and production efficiency. Furthermore, it can also play the role of refining crystalline grain, reducing segregation and improving internal quality. During the reduction with liquid core, the uniform temperature and internal liquid core flow lead to different deformation and stress distribution at various position of shell. The internal liquid flow and external shell interact, so the thin slab casting with liquid core reduction is a thermal-fluid-structure coupled process.In the present thesis, the previous research about the slab reduction with liquid core was reviewed and summarized. Considering the thermal conductivity, liquid core flow and shell deformation, the fluid-structure interaction method was put forward to analyze the stress and strain distribution characteristics and liquid core flow state combining the material high temperature compress experiment and thermal-fluid interaction calculation.In order to improve the simulation precision and make the technological rules for liquid core reduction, the test samples along columnar crystal direction were selected to perform the high temperature compress experiment and analysis under different temperature and strain rate separately.The rheological behavior and influence temperature and strain state to the rheological stress of material were analyzed. The yield stress change model of material thermal deformation and relation of stress and strain were studied too.The three dimension model for mold and secondary cooling zone was set up and utilize finite control volume method to discrete the thermal-fluid coupling equation. The Smei-Implicit Method for Pressure-Linked Equation (SIMPLE) was used to correct pressure-velocity and solve the coupling equation of the pressure. Through analyzing the internal steel velocity field and temperature field, the essential feature of temperature field, the characteristics of them were got in the mold and secondary cooling zone. Meanwhile, the influence of casting velocity, nozzle immersing depth on the flow field and temperature filed were implemented. So initial and boundary conditions were got for liquid core reduction by the thermal-fluid interaction method. Considering the interaction between the liquid core and solidification shell, the fluid-strcuture interaction mechanical theory was used to develop a suitable numerical simulation method for liquid core reduction. The method adopted the dividing area method including fluid domain and structure domain. The fluid domain adopted Taylor-Gelerkin finite element and Reyonlds time averaged technology to solve the incompressible internal liquid core. The structural domain adopted Update-Lagrangian finite element and Newmark step by step integration technology to sovle the solidification shell deformation considering geometry non-linear influence. Thus, the theory model for liquid core reduction has been established. Utilizing the material rheological model got by the high temperature compress simulation experiment and boundary conditions to set up the fluid-structure interaction model for liquid core reduction based on the dividing area method. Through the calculation, the characteristics and rules of shell deformation were got during the liquid core reduction. The distribution rules of stress and strain were also analyzed. In order to understand the liquid core reduction process, the influences of shell thickness and reduction ratio to the reduction result were studied. Meanwhile, the backward flow of liquid core was also investigated using the fluid-structure interaction method during the liquid core reduction. Finally in this paper, the response surface was utilized to fit an approximate equation to describe the technological parameters to the reduction result. In order to optimize the process legibly, a ratio of stress value and reduction per distance was used to evaluate the reduction quality. The inherit algorithm was used to optimize the approximate equation to seek the best technological parameters combination for the slab casting with the liquid core reduction. Through the technological design and optimization, offer scientific theoretical foundation for liquid core reduction.