Research on the cleanliness of 50CrVA spring steel produced by the BOF-LF-CC process

The changes of total oxygen(T. O.) and N contents,the evolution of non-metallic inclusions,and the characteristic of macro-inclusions in casting billets of 50 CrVA spring steel produced by the BOF-LF-CC process were studied by continuous sampling at different stages. Results show that the average contents of T. O. and N are 106×10^-6 and 13×10^-6 at LF-start,respectively,and they become 15×10^-6 and 39×10^-6 in billets,indicating that the deoxidization effect in the LF refining process is obvious. However,reoxidation phenomenon is obvious in the transportation and casting processes,so it is necessary to strengthen the steel stream protection from ladle to tundish. The CaO-Al₂O₃-MgO and CaO-Al₂O₃-SiO₂ composite oxide inclusions are the main inclusions in billets,the Al₂O₃ content in those inclusions is as high as 60%-70%,and no low-melting inclusion is obtained. The low-melting inclusion can be obtained by increasing the slag basicity or feeding a proper content of Ca wire. Large-size inclusions are primarily composed of CaO and CaOAl₂O₃-SiO₂-(MgO) spherical oxides as well as a certain proportion of pure Al₂O₃ inclusions. It is necessary to extend the soft stirring time in the ladle to make inclusions float fully.