A 3D coupled model was developed to investigate the effect of transverse water distribution in the secondary cooling zone on the solidification end shape and centerline segregation of a 450 mm ultra-thick slab. The model’s accuracy was validated by comparing the measured solidified shell thickness, surface temperature, and solute distribution with simulation results. The results show that an improper cooling regime causes uneven solidification along the centerline, forming a W-shaped solidification front, which significantly influences solute distribution. Under the original Case 1 conditions, the maximum solidification position differences ( \({\Delta L}_{{f}_{\text{l}}=0.3}\) and \({\Delta L}_{{f}_{\text{l}}=0.7}\) ) were 1.96 and 2.26 m. For Cases 2, 3, and 4, \({\Delta L}_{{f}_{\text{l}}=0.3}\) decreased to 0.77, 0.42, and 0.39, while \({\Delta L}_{{f}_{\text{l}}=0.7}\) reduced to 0.86, 0.45, and 0.42 m. Centerline segregation was mitigated, with the maximum positive segregation decreasing from 1.37 to 1.23, 1.21, and 1.20. In the straightening segments, the edge temperature ranges for Cases 1 to 4 were 1152.8 K to 1260.1 K, 1121.5 K to 1154.6 K, 1111.4 K to 1123.6 K, and 1110.3 K to 1121.8 K. Cases 3 and 4 showed edge temperatures within embrittlement Zone III (990 K to 1120 K), increasing the risk of transverse cracks. In summary, Case 2, which adjusts the water flow density at the arc segments (Zones 5 to 11) edges to half of that in the middle—can ensure that the surface temperature of the slab edge in the straightening segments remains above the brittle zone III, thereby reducing the risk of edge transverse cracks. Moreover, this configuration substantially improves the terminal W‑shaped solidification morphology ( \(\Delta L<1.0\) m), establishes optimal conditions for mechanical soft reduction, and significantly mitigates centerline segregation in ultra‑thick slabs.
