Effect of LF Refining and Heating on Inclusion Evolution and Coarsening Suppression in Rare Earth Rebar

To investigate the evolution law of inclusions in HRB400E rare earth-treated threaded steel continuous casting billets during the heating process, this study utilized scanning electron microscopy (SEM) and an automated inclusion analysis system (OTS) for comparative analysis. The research evaluated the type, size, distribution, and transformation behavior of inclusions after heating in billets produced through two different processes: direct rare earth addition without ladle refining versus rare earth addition following Ladle Furnace (LF) refining. The results show that LF refining significantly reduces the sulfur content in steel (from 0.032% to 0.007%), and the area ratio of MnS (inclusion area/scanning field area) decreases from 0.092% to 0.010%. After heating, the MnS content in the steel with direct rare earth addition without refining increases significantly, whereas in the steel subjected to LF refining before rare-earth addition, the MnS content increases only slightly. This suggests that LF refining effectively mitigates the tendency for MnS coarsening. During heating process, oxide inclusions (such as rare earth oxides and silicates) in the unrefined samples coarsen significantly, whereas the growth tendency of inclusions in the refined samples decreases. LF refining can improve the comprehensive properties of rare earth threaded steel by optimizing the types of inclusions, refining their sizes and achieving uniform distribution.