OS11(6)-25(OS11W0094) Study on Service Performance of Automobile Steel Sheets

Abstract

Materials with the biggest increase in usage of light-weighting cars and trucks aren't aluminium, magnesium or some other alternative metals. Instead, it is the high strength steel sheets, such as extra-low carbon steels, that have shown the most growth in the light-vehicle market. The continuing emphasis on vehicle mass reduction has driven researchers to investigate the use of thin, high-strength steels in automobiles. Reducing weight by using thinner high strength steel sheets allows the car companies to meet reduced fuel requirements without compromising the size and safety of the cars as well as the buyer's affordability. Extra-low carbons steels, commonly known as interstitial Free (IF) steels have been widely used for automobile plates. IF-steels have also been widely sued in many kinds of products by press deformation particularly of stamped components in automobile application because of their excellent deep-draw ability and high resistance aging. In this study the steel sheets have been prepared by adding Sn (tin), using different content varied from 0.002% to 0.22% mass to other chemical compositions such as Carbon (C), Silicon (Si), Manganese (Mn), Phosphorus (P), Sulfur (S), Aluminium (Al), Nitrogen (N) with almost the same mass percentage. The materials were initially hot-rolled, then cold rolled to 1.4mm thick steel sheets and annealed at 650℃ and 750℃ for 1 minute in laboratory condition by using Batch type furnace. Sheet materials were cut and machined to 30×90mm specimen's longitudinal side parallel with the rolling direction. Curve with 40mm radius was added to each specimen in order to provide stress concentration part for limitation of crack initiation area. All specimen have been tested using the plain bending fatigue-testing machine at 30Hz with capacity of 1.96 Nm. Successive replica observations are performed to observe the fatigue crack initiation and fatigue crack propagation behavior. In addition, the scanning microscope (SEM) was employed for fracture surfaces observation. The main results in this test show that the fatigue strength by 1×(10)^7 cycles increased with increasing Sn (tin) content. The fatigue cracks initiated from the surface and propagated in both intergranular and transgranular mode. The fatigue crack growth rate decreased with increasing tin contents.