Study on nonmetallic inclusions in clean Cu-P-RE weathering steels

Abstract

The compositions, morphologies and sizes of nonmetallic inclusions in rare earth (RE) weathering steels with different oxygen and sulfur contents were analyzed by using electron probe micro-analyzer (EPMA), quantitative image analyzer and scanning electron microscope (SEM-EDS). The formation of inclusions was studied by the thermodynamics theory. The RE solid-soluble content in steel was analyzed by non-aqua electroanalysis and inductively coupled plasma (ICP). To RE weathering steels with different contents of oxygen and sulfur, the optimal RE content was obtained from the perspective of RE modifying inclusions. The results showed the cleanliness of steel was an important factor which influenced the modifying effect of RE on the inclusions. RE changed the character and shape of inclusions in the clean Cu-P weathering steels. The elongated MnS inclusions and brittle oxide inclusions were modified as the small globular RE oxysulfide and RE compound inclusions in the RE weathering steels. The RE inclusions distributed uniformly in matrix and the sizes of more than 90% of the inclusions were less than 2 μm. It was obtained the optimum content of RE was 0.0065 wt% ∼ 0.016 wt% to the Cu-P-RE weathering steels with 0.002 wt% oxygen and 0.004 wt% sulfur, and the optimum content of RE was 0.0092 wt% ∼ 0.023 wt% to the RE weathering steels ( O: ∼ 0.007 wt%, S: ∼ 0.008 wt%). The RE content was the most favorable to modify the harmful inclusions and improve the properties of Cu-P weathering steels.