Kinetic Model to Investigate the Effect of Cooling Rate on δ-Ferrite Behavior and Its Application in Continuous Casting of AISI 304 Stainless Steel

The prediction of M-shaped δ-ferrite content along the thickness direction of continuously cast 304 austenitic stainless steel slab was performed using a diffusion-controlled phase transformation module of Thermo-Calc software. The surface and center of the slab, which solidified with different cooling rates and secondary dendrite arm spacing, were used for kinetic calculations. Comparison between observations and calculations for slab specimens demonstrated that the moving-boundary model could predict retained δ-ferrite content according to various thermal histories. After this validation, the effects of different cooling rates at liquid and solid-state were analyzed to understand the importance of each stage on the diffusion-controlled δ \(\to \gamma\) phase transformation. Decrease in solid-state cooling rate generally had a larger effect on the reduction of δ-ferrite during cooling when the solidification rate was fast (27.75 °C/s) than when it was slow (0.075 °C/s). This model can be used to predict and control δ-ferrite behavior under different thermal histories during slab casting and welding practices.