Inclusion variations of die steel H13, including changes of species, morphologies, compositions, amounts and sizes, in the production of EAF-LF--VD-ingot casting-electro slag refining (ESR) procedure, were investi- gated by systematic sampling, and analyzed with scanning electron microscope (SEM), energy dispersive spectrum (EDS) , and metallographic microscope. The variation mechanism was studied by comprehensive analysis of total oxygen, nitrogen, and acid soluble aluminum as well as chemical test of refining slag. Based on the investigations, technical measures for cleanness improvement were discussed. The resuhs show that oxide inclusions in H13 steel change from irregular Al2O3- near globular CaO-MgO-Al2O3 and CaO-Al2O3-SiO2 complex inclusions-finer CaO- Al2O3-SiO2 inclusions with higher CaO content-CaO-Al2O3-SiO2 inclusions with higher Al2O3 content and irregular MgO-AI203 inclusions--fine irregular MgO-Al2O3-CaS inclusions in various steps of the production; the variations are related with changes of acid soluble aluminum content, reactions between slag and steel, re-oxidation of liquid steel during casting, and refining of ESR. It is also found that Al2O3 inclusions are modified by refining slag in LF and VD refining~ and ESR plays a good role in inclusion removal, especially in controlling the large linear VC-CrC- MoC inclusions distributed in grain boundaries. It is suggested that casting protection should be improved, and the basicity of refining slag and acid soluble aluminum content in steel should be raised.
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