Influence of the Strain Rate on the Mechanical and Technological Properties of Steel Sheets

Increasing of strain rate during forming of semi products or products is one of the ways of production intensification. Therefore an attention is paid to the study of the influence of the strain rate on the material behaviour in the deformation process, but also on the methodology of evaluation of formability at increased strain rates (including impact) 1,2 . In general, it applies that increased strain rates result in increased strength characteristics of materials, while the yield stress is rising up more intensively than the tensile strength 3 . As a result, increased strain rate results in an increased Re/Rm ratio, and for certain materials is ratio Re/Rm > 1. This fact significantly influences the formability (especially compressibility) of materials due to of localization of plastic deformation to “suitable” areas. Plastic deformation is characterized by the fact that its development is markedly non-homogeneous. The degree of non-homogeneity is a function of internal and external factors. The internal factors are internal mikrostructure of material, as follows: number and structure of phases, grain size and structure type. By increasing of grain size and number of phases, non-homogeneity of plastic deformation significantly increased. The temperature, strain rate and the stress state are crucial external factors 4,5 . The sensitivity of materials on strain rate during the forming process is, as it was mentioned earlier, a function of material, and therefore it is beneficial to analyze this sensitivity, especially for new developed materials intended for the cold forming. This is necessary in order to determine the limit state, as well as the properties of the final product. The main aim of the paper is to extend the knowledge and to mention on certain problems occurring during forming at increased rates (up to impact loadings).