Avaliação dos efeitos da pré-deformação e do envelhecimento nas propriedades mecânicas de tração de chapas de aço USI BH 180

Abstract

Bake Hardening (BH) steels are widely studied and applied in the automotive industry, as an excellent choice for balancing formability, weight and mechanical strength. Galvanized USI-BH 180 steel sheets are used in stamping operations of trunk lids, hoods and other parts of the metallic structure of automotive vehicles, which have a yield strength in the range of 180 to 240 MPa, also have resistance to tensile strength in the range of 300 to 360 MPa, a minimum elongation of 34%, and acquire their final mechanical properties after mechanical forming and the painting cycle, where temperature accelerated aging occurs. Thus, in addition to the works found in the literature, the present dissertation aims to evaluate the effects of pre-strain and accelerated aging on the mechanical tensile properties of laminated USI-BH 180 steel sheets, varying the amount of area reduction (2% and 5%) at temperature between 150-200ºC, respectively. Specimens will be made in accordance with ASTM 8M standard. To obtain the planar and normal anisotropy indices, the mechanical properties of specimens located in the rolling direction, perpendicular to the rolling direction and 45º in relation to the rolling direction will be evaluated. To relate to the mechanical tensile properties, the different samples will be obtained measurements of Vickers microhardness. Microstructural information will be obtained from optical microscopy, scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. It was noted in the tensile results performed after accelerated aging, that the yield strength showed a significant increase for all samples tested, when compared to the tensile results performed on the specimens without accelerated aging. A significant increase was observed for Vickers hardness measurements. Microstructural characterization analyzes were carried out, through optical microscopy and x-ray diffraction methods, where the necessary information was obtained to classify the material as single-phase of ferrite matrix, and it was possible to analyze the mechanisms of fractures existing in the ruptured samples, with the aid of the scanning electron microscopy method. Thus, the development and results of this work aim to contribute to the understanding of the effect pre-strain and accelerated aging step of US steel I BH 180, as well as contributing to the knowledge of the microstructure of the steel in question. Finally, the work also aims to contribute to the national industry in the development of automotive steel.