Effect of Rare Earth Element La on Microstructure and Properties of Q355B Low Alloy High Strength Steel

Q355B low alloy high strength steel containing rare earth element La with mass fraction of 0.002 1% was prepared by vacuum melting method. The microstructure, impact and tensile properties and the corrosion resistance in NaHSO₃ solution of the test steel were studied, and were compared with those of the test steel without rare earth element La. The results show that the microstructure of Q355B steel without and with La was ferrite and pearlite, and the addition of La decreased the element segregation degree of the structure, reduced the band structure, and improved the uniformity of grain size. The test steel without La had angular string Al₂O₃ inclusions with the size of about 267 μm; after adding La the inclusions were spherical rare earth complex sulfur oxides, and the size significantly decreased to about 10 μm. The addition of La had no significant influence on the tensile properties of the test steel, but could improve the impact properties. Compared with that of the test steel without La, the impact absorption energy after adding La increased by 5.2%. Compared with those of the test steel without La, the corrosion mass loss rate of the test steel with La was reduced by 5.5%, the corrosion product layer was denser and the corrosion resistance was better.