Numerical model of controlled cooling in production of steel hot rolled bars was developed. By numerical model of controlled cooling is possible to predict a transient temperature field, microstructure evolution and hardness of rectangular steel bars during their cooling in cooling beds. The numerical model of transient temperature field is based on control volume method. The algorithm for prediction of hardness and microstructure distribution in steel bars is based on continues cooling transformation, (CCT) diagrams and real chemical composition. The numerical model and algorithm is completed to solve problems in controlled cooling of hot rolled bars in cooling beds. The controlled cooling are performed by special placement of hot rolled bars on cooling beds. Numerical model and computer program was experimentally verified by simulation of real industrial production of low alloyed steel bars. The verification of developed numerical model was performed by comparison of simulated hardness with experimentally evaluated results.