Deformation Behaviour of C-Mn440 Automobile Steel under Varying Strain Rates.

Abstract

The prime objective of vehicle designers is to produce vehicles with fuel-economy along with the safety standards imposed by the government and stringent consumer demands. It is known that reducing the weight of a vehicle is a straight forward strategy to improve fuel-economy, but it can potentially create safety problems which in turn leads to increased utilization of high strength steel sheets for the automobile body components. C-Mn and DP steels consist of outstanding combination of both strength and ductility. High strain rate experiments are generally used to study the material behaviour when these are subjected to high speed impacts, like crash. The mechanical behaviour of these materials at high strain rates is considerably different from that observed at quasi-static loading because of the strain rate sensitivity of the material. Hence, automotive industries are continuously engaged with designing newer materials for car body applications. In this investigation, many quasi static tensile experiments were carried out at various strain rates on C-Mn 440 steel at ambient temperature. It has been observed that yield and tensile strengths of the material increase drastically while %EL and %RA significantly decrease with the strain rate. The fracture surface reveals dimple morphology with variation of dimple geometry with strain rate; average dimple diameter increases and dimple density decreases with strain rate. The void accumulation (i.e., void density) inside the material increases with the increase in true strain for all the strain rates. At the initial stage of strain, void density increases slowly and at the later stage, void density increases rapidly. The dislocation densities in all deformed specimens have been calculated from X-ray diffraction profile analyses using modified Williamson-Hall method; the results show that dislocation density increases with strain rate.