The copper mould and molten steel were modelled simultaneously by a three-dimensional (3D) model, which couples fluid flow, heat transfer and solidification. Thermal boundary conditions (e.g. air gap distribution, slag resistance, contact resistance) along casting direction and circumference direction were discussed and applied. The results of solid shell thickness distribution were compared with reference experimental data and reasonable agreements were found. The calculations indicate that, the shell thickness distribution along circumference in web was affected by the cross-section sizes. The uniform shell thickness distributions at narrow face and R corner were caused by the scouring of fluid flow. The heat transfer fluxes at hot face of mould were various at different locations.

This paper mainly focuses on the problem of coarse grain on the bottom surface of the red copper horizontal continuous casting billet, and tries to thicken the bottom end of the crystallizer by combining numerical simulation and industrial experiment, and indirectly improves the cooling strength of the bottom surface of the casting billet to solve the problem of grain coarseness. The results show that the optimum diameter of the lower inlet of the copper sleeve was 14.2 mm, when the diameter of the inlet increased from 11 mm to 14.2 mm, the cooling intensity of the lower part of the casting billet gradually increased, and the grain on the lower surface was significantly refined; and with the increase of the diameter of the water inlet, the average width of the columnar grain decreased from 1.94 mm to 1.64 mm, the grain size decreased, and the number of grains increased.