Analysis on inclusions in bearing steels

Inclusions in hot rolled sheet of bearing steel and blockage in clogged submerged entry nozzle(SEN) were analyzed using scanning electron microscopy(SEM). It is found that the inclusions in hot rolled sheet are mainly TiN,MgO·Al₂O₃,MnS,and a little CaS,most of them are smaller than 20 μm. Besides,some large inclusions of MgO·Al₂O₃ and Al₂O₃ and the associated cracks are observed. By the detection on clogged nozzle,it is shown that the blockage is mainly MgO·Al₂O₃ spinel inclusions as well as some MgO-Al₂O₃-CaO system inclusions and solidified steel. The composition of blockage is consistent with that of inclusions in steel,thus it is indicated that the nozzle clogging is caused by the gathering and precipitation of solid oxide inclusions in molten steel on SEN during the continuous casting process. A possible reason for the existence of large inclusions in sheet may be the drop of blockage during casting.