Morphology simulation and mechanical analysis of primary dendrites for continuously cast low carbon steel

Abstract

The initial growing dendrite is influenced significantly by the complicated solidification conditions in continuously oscillating mold. The uneven growth of dendrite causes some defects seen commonly such as internal crack, subsurface porosity, subsurface inclusion and other defects of continuous casting billet. The induced initial defects in mold can be expanded and propagated in the following process such as secondary cooling, straightening, rolling and other subsequent handling procedure and then evolve into serious defects that can restrict the development and the quality refinement of final steel products. The mechanical stress caused by mold oscillation and the melt flowing is a crucial factor that leads to the uneven microstructure growth of initial solidifying shell in continuous casting mold. In this work, we simulate the growth and the morphology evolution of primary dendrites in mold area by using the cellular automaton (CA) method in combination with the actual conditions for continuously cast low carbon billet (Fe-0.6 wt.%C). Further, the mechanical state of initial dendrite is analyzed by regarding primary dendrite as a cantilever beam and its mechanical stress is calculated by combining thermo-physical properties and flow rate of steel based on the principle of materials mechanics to shed light on the formation of initial defects formation in mold area of continuous casting process. The results show that the solute concentration of initial dendrite tip gradually increases with undercooling from 2 to 10 K, and the maximum concentration rises by 0.07% when the increment of undercooling is 2 K. The length of dendrite arm increases significantly with undercooling from 2 to 6 K. However, the length of dendrite arm remains steady in a stable growth rate of 0.08 mms-1 when the undercooling is enhanced from 6 to 10 K. The increase of undercooling reduces the bending stress at dendrite root when the flow rate of molten steel is improved from 0.13 to 0.33 ms-1, while the mechanical stress continuously increases with the growth of primary dendrite at a constant undercooling. The bending stress of dendrite root has a high possibility to exceed its critical fracture strength under the condition of undercooling below 6 K or dendrite grow up more than 1 s. The primary dendrite is likely to be fractured and form initial defects of billet.