Effect of O/S and Ce/La ratio on the occurrence state of inclusions in U75V steel

Abstract

Non-metallic inclusions in steel have a significant impact on the comprehensive properties of the steel. Although it is not possible to completely remove these inclusions, their impact on mechanical properties can be mitigated by controlling their morphology, quantity, and other characteristics. This study focuses on the oxygen-to-sulphur (O/S) ratio and the lanthanum-to-cerium (Ce/La) ratio as key factors and designs an orthogonal experiment with three factors at four levels. Using FactSage 7.1, the composition of inclusions in steel was calculated for 80 sets of compositions, analysing the state of inclusions in U75V steel under different O/S and Ce/La ratios. Within the national standard composition control range for U75V steel, when the total rare earth content in the steel is less than 0.015% and 0.045 < O/S < 0.075, if Ce/La > 3, the inclusions in the steel mainly exist in the form of Ce₂O₂S, La₂S₃, Ce₂S₃, and MnS; if 1 < Ce/La < 3, and if the O/Al ratio in the steel is greater than 0.52, Ce₂S₃ is replaced by Al₂O₃; if Ce/La < 1, Ce2S3 does not exist in the steel. When the O/S ratio in the steel is greater than 0.5, and the total rare earth content reaches 0.015%, MnS inclusions completely disappear, and the sulphides in the steel are mainly rare earth sulphides and oxysulphides; for 0.5 < O/S < 2, and Ce/La > 3, the inclusions in the steel mainly exist in the form of Ce₂O₂S, La₂O₂S, Ce₂S₃, and La₂S₃; for 0.5 < O/S < 2, and 1 < Ce/La < 3, if the O/Al ratio in the steel is greater than 3, Ce₂S₃ is replaced by Al₂O₃; for 0.5 < O/S < 2, and Ce/La < 1, the inclusions in the steel mainly exist in the form of Ce₂O₂S, La₂O₂S, Ce₂S₃, and La₂S₃, with Al₂O₃ disappearing. To verify the thermodynamic predictions of the state of inclusions in U75V steel under different O/S and Ce/La ratios, vacuum induction melting was used to smelt U75V steel containing lanthanum and cerium. Sample 2# had an O/S ratio of 1.05 and a Ce/La ratio of 0.514, with inclusions mainly existing as Ce₂O₂S and La₂O₂S; Sample 3# had an O/S ratio of 1.33 and a Ce/La ratio of 0.115, with inclusions also mainly existing as Ce₂O₂S and La₂O₂S, confirming the computational results of the thermodynamic predictions for the state of inclusions under different O/S and Ce/La ratios.