Abstract: Taking 45 steel bloom as object for the study, a solidification heat transfer model has been built. The pair-wise mixture model was used to calculate the thermal parameters of 45 steel. Besides, actual temperatures and shell thicknesses of narrow face were measured by pin shooting method and infrared pyrometer to verify the correctness of heat transfer model and the results indicate the model could simulate the heat transfer process of 45 steel well. Simulation results have showed that appropriate position of soft reduction was 21.8－23.1 m from the meniscus at the condition that superheat was 35 ℃ and casting speed was 0.5m/min. And the amplitude of rising temperature on the surface of bloom after going out of the mould was huge, which easily leads to surface cracks. Steadying the casting speed was the key factor for improving quality of 45 steel. After the application of those results, centre porosity grades have decreased from 1.5to not more than 1.0; centre shrinkage grades were not more than 0.5;the largest carbon segregation was not more than 1.12；rates of surface cracks has dropped from 3.25 % to 0.5 %. As for the accelerated solidification at the end of solidification of bloom, that’s because of the rapid growth of equiaxed grains in the slab center with a great undercooling for liquid steel.

Key words: heat transfer of solidification, solidification rate, numerical simulation, 45 steel