The main task of the technology of the steel sheet flattening is to calculate the optimal reduction of a sheet billet by the working rollers of straightening machines so that the sheet at the outlet from the machine has the minimum residual stress and curvature. In the mathematical and numerical modeling of the flattening process of the steel sheet in the multiroll straightening machines, in the beginning we calculate the curvature and bending moments of the steel sheet at the points of the tangency with the machine’s working rollers, and then we calculate the energy-power parameters of the sheet’s flattening. The calculation of energy-power parameters of the multiroll sheet-straightening machines is an important technological estimation at the steel sheet’s flattening. The basis of energy-power calculation includes the estimation of the support reactions of working rollers and the efforts of the upper and lower rollers’ cassettes of straightening machine at the sheet flattening. When there is an insufficient bending moment of steel sheet, it is impossible to eliminate the harmful residual stresses in the sheet wall and the surface defects of the sheet. If the force of the upper cassette rollers is insufficient, then to achieve the required reduction of the sheet for the quality flattening is impossible. The excessive values of the rollers’ torque moments and the efforts of rollers’ cassettes often lead to the sheet defects, the breakage of the working and supporting rollers and the breakage of whole sheet-straightening machine. The approximate method for the determining of the optimal technological parameters of the cold flattening of the steel sheet on the 2N-roller sheet-straightening machine is proposed in this paper. The calculations allow us to determine the type and curvature of the neutral plane of the steel sheet under the flattening, the residual curvature of the sheet after the flattening, the sheet’s bending moments, the support reactions of working rollers, the residual stresses in the wall of the steel sheet, the proportion of plastic deformation on the sheet thickness and the relative deformation of the longitudinal surface fibers of the sheet under the flattening depending on the rollers’ radius, the pitch between the straightening machines’ working rolls, the magnitude of the sheet reduction by the upper rollers, the sheet thickness, as well as the elastic modulus, the yield stress and the hardening modulus of the sheet metal. The research results can be widely used at the engineering and metallurgical plants.