ABSTRACT

Extra-low carbon steels have been widely used for automobile plates, etc. Compositions of materials influence most of material properties. In the present paper, fatigue properties of extra-low carbon steels with different tin contents, varied from 0.002 to 0.22% mass, are investigated. The results show that the fatigue strength by 1 × 107 cycles increases with increasing tin concentration. The cracks initiated from the surface of specimen and propagated mainly in intergranular modes for all materials. However, the number of transgranular cracks increase with increasing tin contents. Moreover, the crack growth rate was found to decrease with increasing tin contents.

INTRODUCTION

Extra-low carbon steels, commonly known as IF (interstitial free) steels, exhibit excellent fabrication characteristics such as high rollability, low deformation resistance, fine surface finishing, excellent weldability and high machinability. Therefore, these materials have been widely used in many kinds of products by press deformation particularly of stamped components in an automobile application. The main purposes of manufacturing extra-low carbon steels, which are recently being developed by avoiding interstitial elements such as carbon or nitrogen and adding substitutional elements such as manganese or silicon, are to compensate for the decrease in the material strength and the improvement of the drawability (Butler, 1962). In many failure types, Nishida (1992) stated that more than 90% of mechanical failures in steel structures have been affected directly or indirectly by fatigue. Furthermore, it has been found that more than 90% of fatigue cracks were initiated from locally high stress concentration parts. Also, the grain flow direction in the rolled sheets related to the direction of loading has a significant effect on fatigue strength. Collins (1981) stated that fatigue strength of cyclic loading across the grain direction (transverse) is lower than that of loading along the grain direction (longitudinal).

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