Crack Analysis of 44SMn28 Steel Bar for Automobile Connecting Rod

In view of the frequent appearance of surface and internal cracks in the production of automobile connecting rod steel 44SMn28, based on the combination of pickling macro-detection, optical microscope, scanning electron microscope observation and energy dispersive spectroscopy analysis, the cracks of 44SMn28 rolled bar for peeling billet and black rolled billet were observed, and the precipitation, morphology, distribution of sulfide and the crack generation mechanism were also studied. The results show that there are a large number of MnS inclusions in the steel, ranging from hundreds of nanometers to hundreds of micrometers in size, distributed in granular, spindle or strip shape within the grain and grain boundaries. Banded sulfide is the main cause of cracks in steel, and its anisotropy significantly reduces the transverse mechanical properties of the materials, resulting in the initiation of cracks along its interface under stress. There is also a lot of oxide in the crack, and there is a lot of FeS between the oxides and the matrix, which is peculiarly prone to cause steel cracking under the action of thermal stress and mechanical stress. Compared with the peeled rolling materials, the surface cracks of the black rolled materials are significantly reduced, the crack depth is shallow. Moreover, the internal cracks of rolled materials are more likely to propagate along the grain boundary in the case of low temperature deformation.