Abstract: In the current study, the optimization experiment of VD bottom blowing process for Q345D wind power steel was carried out to improve the removal efficiency of large-size inclusions in VD refining process. Before the VD vacuum treatment, inclusions in the steel were calcium aluminate with high Al₂O₃ content under the bottom blowing condition of 400 L/min in the first 10 min and 250 L/min in the last 5 min during VD high vacuum process (scheme 1), while the average composition of inclusions in the optimized process was closer to liquid calcium aluminate under the bottom blowing condition of 500 L/min in the first 10 min and 150-100 L/min in the last 5 min during VD high vacuum process (scheme 2 and scheme 3). Inclusions in the steel after VD treatment were all low melting point. For scheme 1, >10 μm and >15 μm inclusions increased by 66.24 % and 62.32 % respectively. However, >10 μm and >15 μm inclusions in scheme 2 increased by 23.40 % and 33.39 % respectively, especially the number density of inclusions increased by only 16.33 % and 28.57 % respectively. It was indicated that a strong stirring in the first 10 min and a moderate stirring in the last 5 min during VD high vacuum process could promote the removal of large-size inclusions and reduce the probability of slag entrainment.

Key words: VD refining, bottom blowing process, large-size inclusions, wind power steel