The evolution of inclusions in high quality GCr1 5 bearing steels during secondary refining process
Based on the productive process of BOF-LF-RH-CC for GCr1 5 bearing steels and using FE-SEM/EDS,evolution rule and mechanism of the inclusions in LF and RH refining process have been studied.Results show that two types of evolution manners of the complex inclusions for the steel during refining process are summarized.The complex inclusions originate from Al2 O3 and then change to MgO· Al2 O3 → (CaO-MgO-Al2 O3-(CaS )) and (Al2 O3-MnS )→ (Al2 O3-MnS- Ti (C,N )) respectively. Desulfurization effect is obvious in LF refining process, and sulfide inclusions in steel decrease significantly.The main inclusions at the beginning of LF refining process are some simple inclusions such as MnS,Al2 O3 ,TiN,etc.At the end of LF refining process,the inclusions are compounds which Al2 O3 is covered with MnS,TiN,Ti(C,N)or CaS on the surface.CaO in refining slag and MgO in refractories are easily reduced by aluminum and carbon in the steel and then Ca and Mg will react with the dissolved oxygen in steel,which leads to the number of D-type inclusions increasing at the end of LF refining.After RH and soft blowing process,sulfide inclusions are removed further,the number of D-type inclusions and complex D-type inclusions covered with A-type and T-type inclusions increase.During the LF refining,the size of inclusions mainly concentrates in the range of 1-3μm.During the RH refining,the size of inclusions mainly concentrates less than 1μm.The maximum size of the inclusion changes from 10.79μm to 5.68μm,and the number of inclusions in per square microns decreases from 372/mm2 to 258/mm2 .RH and soft blowing process have removed the inclusions larger than 3μm in steel effectively.
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