The liquid steel flows along the casting direction was ignored in this model. Therefore, the simulation of strand in secondary cooling zone was simplified to a two-dimensional heat transfer model. The latent heat of solid phase transformation was ignored as well.

(b) Computational Domain

The 2-D model is presented in Fig. 2. It is also a quarter model in consideration of the symmetry. This model moves at casting speed from mold exit to secondary cooling zone exit to compute the heat transfer during the whole secondary cooling process.

(c) Boundary Condition

The heat flux boundary condition was applied at strand surface, which included the heat transfer of water impact, radiation and roller contact. According to actual continuous casting process, the strand was divided into 13 regions along the casting

Fig. 2 The schematic diagram of the 2-D model

Table 2 The physical properties of steel

Physical properties

Values

Density (kg/m^{3})

7200

Specific heat (J-(kg-K)^{-1})

700

Thermal coefficient (W-(m-K)^{-1})

61.46-0.02 x T(K)

Viscosity (kg-(m-s)^{-1})

0.0062 [10]

Latent heat (J/kg)

264300

Solidus temperature (K)

1736

Liquidus temperature (K)

1787

direction. In addition, the strand was divided into 5 regions along the width direction to simulate the non-uniform secondary cooling water density.

Physical Properties

In the simulation, the physical properties of steel are presented in Table 2.

Solution Procedure

In this study, the CFD software FLUENT 15.0 was employed. The 3-D model was solved at a time step of 0.05 s for 350 s to make sure all residuals were below 1 x 10^{-3} and the average temperature on the outlet was steady. Then the temperature field at mold exit was exported to be the initial condition of the 2-D model. The 2-D model was solved at a time step of 0.5 s for 1740 s to compute the whole secondary cooling process.