Inclusion formation and evolution in steelmaking process for drill pipes

The crystallization and evolution of non-metallic inclusions in CrMo drill pipe steel refined with a complex deoxidizer were investigated for the EAF-LF-VD-CC process flow of a domestic steel plant. As a result of aluminates floating, lower TO content in the steel before the LF refining was shown, while nitrogen content in the steel gradually increased during the whole flow. Large size inclusions (about 50 μm), namely, silicic manganates with low melting point, complexes with Na₂O and magnesium aluminate spinels with a little CaO, appeared at the LF-VD stage and disappeared completely after the tundish process. Small size inclusions (< 10 μm) could be found during the whole refining flow. Their size at different refining stages was similar to each other, while smaller than that of the casting blank. The small size inclusions were complex oxides which consist of Mg, Al, Si, Ca and CaS respectively. The composition of the small size inclusions gradually moved to the liquid phase region during the smelting flow, and competition existed between CaO and MgO contents in these inclusions. Macro inclusions (> 100 μm) in the casting blank included complex inclusions caused by entrapped slag, MgO-CaO inclusions from refractories and endogenous aluminate inclusions coated by a CaS layer, whose composition was similar to small size inclusions in the smelting processes. The macro aluminate inclusions were easily broken into chain and further resulted in cracks in the steel during the rolling process. Hence, it is suggested to prolong the steel standing time after the VD operation to remove detrimental aluminate inclusions.