Formation mechanism of oversized DS-type inclusions in low oxygen special steel

To investigate the generation mechanism of oversized DS-type inclusions in low oxygen special steel,a comparative analysis of inclusions was performed on overseas and domestic special steel samples by the automatic secondary electron microscopy ASPEX PSEM explorer.Both the average size and maximum size of inclusions are larger in domestic samples than overseas samples.Particularly,the maximum size of inclusions in domestic samples is several times that in overseas steel samples:24.9 and 13.1 μm in domestic samples and 7.6 and 7.5 μm in overseas samples,respectively.Based on the chemical composition analysis of inclusions in domestic samples,it is found that the oversized inclusions and smaller inclusions have similar compositions.Therefore,the detected DS-type inclusions are endogenous but not exogenous,which can be produced by collisions among inclusion particles.To further study the possible origins of the oversized DS-type inclusions,in-situ observation on the behaviors of inclusions at the solid/liquid interface in low oxygen special steel show that tiny inclusions less than 5 μm can be easily and stably captured by the solid/liquid interface with a total oxygen content of 7×10^-6.Moreover,collisions,agglomeration/coalescence and growth of inclusions captured by the solid/liquid interface result in the formation of much larger size DS-type inclusions over >12 μm.