A Study of Network Defects of a Phosphated Si-containing Low Alloy Steel

This study was to investigate the formation of network defects that appeared on a phosphated low alloy steel rod containing 1.1% Si. Laboratory oxidation, pickling, annealing, and phosphating were performed. A scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) was employed to analyze the surface morphology and the elemental distribution of the specimen from each stage. From the results, a mechanism of formation of network defects has been proposed. Fast finish rolling produced fragments of Si-containing iron oxides, which were not removed completely through primary pickling → annealing → secondary pickling. Then, small pits occurred on the substrate near the interface of the substrate and the residual scale. After phosphating, small pits grew to large pits and propagated along the grain boundaries. Some grains spalled off eventually and looked like a network by human naked eyes.