OS11(6)-26(OS11W0284) Effect of Sn Content on Fatigue Properties of Extra-Low Carbon Steels

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The requirement of the high processing high strength light steel sheet recently rises in the car from the global environmental problems and so on from the body lightening because of fuel expenses and the view point of the safety improvement at the time of the collision. Extra-low carbons steels, commonly known as Interstitial Free steels (IF) have been widely used for automobile plates because of their excellent fabrication characteristics such as high formability, low deformation resistance, fine surface finishing and excellent weld ability. IF steels are developed by avoiding interstitial elements such as carbon and nitrogen in order to obtain superior forming properties. Substitution elements such as manganese or silicon are added to compensate the decrease in material strength caused by small carbon contents. The objectives of this study were to investigate the effect of Sn (tin) content on the fatigue strength and fatigue crack behaviors of extra-low carbon steel sheets with different Sn (tin) content varied from 0.002 to 0.22% mass. The materials were initially hot-rolled, then cold rolled to 1.4mm thick steel sheets and annealed at 650℃. Sheet materials were cut and machined to 30×90mm specimen's longitudinal side parallel with the rolling direction. All specimen have been tested using the plain bending fatigue-testing machine at 30Hz with capacity of 1.96Nm. Successive replica observation methods were performed to observe the surface crack initiation and propagation behaviors. The microstructures of each material were observed using optical microscope. In addition, the scanning electron microscope (SEM) was employed for the fracture surface observation. The main results in this test are as follows. (1) The fatigue strength by 1×(10)^7 cycles of extra-low carbon steel increased with increasing Sn (tin) content. (2) The grain size of materials with increasing Sn (tin) contents. (3) The fatigue cracks initiate from the surface of specimen and propagated mainly in intergranular mode for all materials. According to the relation of crack length and number of cycles, the fatigue crack growth rates decrease with increasing tin contents. (4) A non-propagating crack cannot be observed in the specimen subjected under the stress level of fatigue limit by 1×(10)^7 cycles.
2003-09-09

OS11(6)-26(OS11W0284) Effect of Sn Content on Fatigue Properties of Extra-Low Carbon Steels

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