The technology of hot flat rolling is a complex technological process that includes multiple shaping of a steel billet, the temperature of a bullet ranging from 1100 to 900 degrees [1-6]. To deform a workpiece, mill rolls are used. The control over a workpiece thermal state is an important task aimed at the quality of its manufacturing [7-13]. To achieve the goal, a mathematical model is developed for calculating a steel billet thermal state. The model makes it possible to determine a temperature distribution along the whole length of a workpiece. The study results are useful for researchers and developers of the hot rolling technology, and they can be applied in the organization of the rolling process with an allowance for the temperature distribution. Based on the results, the parameters of steel billet production can be adjusted or changed [14-18].