STUDY ON DEEP DRAWING BEHAVIOR OF THE INTERSTITIAL FREE STEEL IN CORRELATION WITH THE MICROSTRUCTURE

Abstract

The Interstitial Free steels (IF) are widely used in the automotive industry due to their excellent mechanical properties (relative to deep drawing), combined with high mechanical strength. Currently, the main applications of the IF steel are focused on automotive industry parts production with a high forming requirement, such as the exterior panels of the cars body. The objective of this paper was to present results obtained about the behaviour of the IF steel sheets associated with high deformation. In order to evaluate the deep drawing aptitude of the sheet, samples were subjected to different mechanical tests: tensile test, hole expansion tests, n and r values determination and Erichsen test, all of them used commonly to verify the deep drawability of the sheets. This information was completed with microhardness measurements. The results were correlated with microstructural studies involving optical microscopy and scanning electron microscopy (SEM), including semi-quantitative EDS analysis. The IF steel microstructure is formed from ferritic grains with very fine grain size. Different types of precipitates are distributed on the grain boundaries and within grains. The information obtained allowed to understand the mechanical behavior in relation with the distribution of the particle's types (identified in the structure), their composition and impact on the microstructure evolution associated with temperature. Through thermodynamic simulation applying FactSage 8.1, the phase transformation temperature (Ty->a) and the conditions of precipitates formation were estimated, in order to correlate with industrial processing conditions of the material. In addition, Ty->a value was determined by dila-tometry test. The main precipitates identified in the microstructure were TiN, TiC, TiS and C2S2Ti4, in coincidence with the results of the thermodynamic predictions. It is known that the failure of the material during the deep drawing or stamping process of the pieces is evidenced through the formation of microcracks or cracks in areas with changes of critical angles. For this reason, it is relevant to understand the mechanisms of fracture nucleation and propagation of the IF steel. In this sense, a fractography study was carried out on the samples tested by tensile test up to failure. It was possible to verify the presence of cavitation mechanism as results of super-plastic flow at high deformation conditions, promoting necking and fracture after a high plastic deformation achieved. All the results obtained allowed to verify that the IF steel sheet satisfy the requirements of an EDDQ quality steel.