This paper describes the on-line implementation and evaluation of a model describing heat transfer in steelmaking ladles. The implemented model is based on ordinary differential equations which ensures a short computation time. Model calibration was performed utilizing a grey-box approach using real process data in order to describe the specific ladles used by a Swedish steel manufacturer. The main purpose has been to design and implement a simulation prototype for on-line simulation in the steel mill. To evaluate the model and the implementation, simulation of 14 steel charges was performed in real-time in the steel mill. Further evaluation of different models and different conditions is needed, although the performed experiment seems promising.