Effect of La content on sulfur-containing inclusions,microstructure evolution and pitting corrosion resistance of sulfur-containing peritectic steel

In this paper, sulfur-containing peritectic steels with different La contents (0 wt%, 0.014 wt%, 0.029 wt% and 0.061 wt%) were studied. The effect of La content on the characteristics of sulfur-containing inclusions was studied by scanning electron microscope and energy dispersive spectrometer. The effect of La content on the microstructure evolution was studied by high temperature confocal laser scanning microscope, optical microscope and X-ray diffraction. The effect of La content on pitting corrosion resistance was studied by polarization curve test. The results show that with the gradual increase of La content (0 wt%∼0.029 wt%), the typical inclusions in sulfur-containing peritectic steel change from MnS and Al₂O₃ + MnS to La₂O₂S + MnS and La₂O₂S-LaAlO₃ + MnS, and then to La₂S₃, the number density of inclusions decreases from 308 /mm⁻² to 210 /mm⁻², the average size decreases from 3.0 μm to 2.4 μm, and the shape changes from irregular to spherical or near-spherical. When the La content is increased to 0.061 wt%, La and P are combined to form LaP inclusions, the number density of inclusions increases to 271 /mm⁻², the average size increases to 2.7 μm, and the shape changes into an irregular shape. When the La content is 0.014 wt%, 0.029 wt% and 0.061 wt%, the austenite grain size is refined from 408.3 ± 9.2 μm to 190.4 ± 5.8 μm, 152.7 ± 5.1 μm and 195.2 ± 5.9 μm, respectively, and the pitting potentials are increased from −0.41 ± 0.01 V to −0.37 ± 0.01 V, −0.34 ± 0.01 V and −0.39 ± 0.01 V, respectively. When the La content is 0.029 wt%, the inclusion characteristics, microstructure refinement and pitting corrosion resistance of sulfur-containing peritectic steel are the best.