Most steelmaking ladle modelling strategies are based either on pure physical modelling or statistical relations. In this paper, a relatively simple model for steel temperature and ladle temperatures based on ordinary differential equations is presented and compared with a number of more complex ODE models. The model is semi-physical since the model structure is obtained mainly from physical relations, but the model parameters are calibrated using collected process data and a systematic calibration methodology. A main objective has been to obtain a model suitable for on-line purposes. Different model expansions are compared to the reference model. It is found that a simple model structure gives a good trade off between accuracy and complexity.