Phase Transformation Modeling for Hypo Peritectic Steel in Continuous Cooling

Phase change of steel during cooling affects the slab qualities in continuous casting. Especially, crack susceptibility of hypo peritectic steel is high because large volume shrinkage occurs by peritectic phase transformation during solidification and cooling. In continuous cooling, phase change is different from the behaviors under the equilibrium condition, such as undercooling and extend of peritectic reaction, etc. Therefore, we develop a new phase change model considering thermodynamics, empirical equations, and carbon diffusion in each phase to predict phase change behavior during continuous cooling. In this model, phase change of hypo peritectic steel comprises 5 stages until all phases become the γ phase. The velocities of the δ/γ interface and phase fractions during cooling are calculated according to cooling rate, undercooling of the γ phase, and carbon contents. The results show that if solidification ends by the δ phase during dT_p, the γ phase is formed by massive transformation. On the contrary, if peritectic reaction starts with liquid, the γ phase is formed and grows by diffusional transformation. In latter case, massive transformation of remaining δ phase can occur with high undercooling or very fast cooling rates. This analysis shows that there are several different paths depending on carbon contents of hypo peritectic steels.

Phase change of hypo peritectic steel.