Inclusion Variations of Hot Working Die Steel H13 in Refining Process

Inclusion variations of die steel H13, including changes of species, morphologies, compositions, amounts and sizes, in the production of EAFLF→VD→ingot casting→electro-slag refining (ESR) procedure, were investigated 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 results show that oxide inclusions in H13 steel change from irregular Al2O3→near globular CaO-MgO-Al2 O3 and CaO-Al2O3-SiO2 complex inclusions→finer CaO- Al2 O3 -SiO2 inclusions with higher CaO content→CaO-Al2 O3 -SiO2 inclusions with higher A12O3 content and irregular MgO-Al2O3 inclusions→fine irregular MgO-Al2 O3 -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 A12O3 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.