Abstract: Q345R steel is widely used in the manufacture of pressure vessels and boilers, which demand high mechanical properties and corrosion resistance because they are usually in service at high temperatures, high pressures or in environments with corrosive media. The presence of inclusions in steel could deteriorate the properties of the steel, seriously affecting the service safety and service life of the equipment. Understanding the formation mechanism and evolution of non-metallic inclusions in the refining and casting process of Q345R steel is a prerequisite for optimizing the inclusions control process. To this end, the evolution of inclusions in the refining and casting process of Q345R steel was investigated by industrial trials, and the influence of Ca treatment, reoxidation as well as cooling and solidification on the evolution of inclusions was discussed with FactSage 8.3 thermodynamic software. The trial results showed that at LF ending to VD breaking, the inclusions in steel were transformed from MgO-Al₂O₃, CaO-Al₂O₃-MgO(-CaS) and a small amount of CaO-CaS to liquid CaO-Al₂O₃-MgO, and the number density of inclusions was reduced from 53.58/mm² to 10.98/mm², and the removal rate of the inclusions was as high as 79.51%. After Ca treatment, the inclusions into CaO-Al₂O₃-MgO(-CaS) and a small amount of CaO-CaS, the mass fraction of Al₂O₃ in inclusions decreased to 35.10%, while mass fraction of CaS increased to 20.61%. During soft blowing to the ladle casting 5 min, the liquid steel occurred a serious reoxidation, the inclusions from CaO-Al₂O₃-MgO(-CaS) and a small amount of CaO-CaS transformed into CaO-Al₂O₃-MgO and MgO-Al₂O₃, the number density increased from 22.59/mm² to 53.77/mm², and the average size decreased from 1.95 μm to 1.56 μm. In the as-cast slab, due to the large amount of CaS precipitation, the inclusions were mainly MgO-Al₂O₃, CaO-Al₂O₃-MgO(-CaS), CaO-CaS and a small amount of MgO. In addition, based on thermodynamic calculations, a new target for inclusions modification during Ca treatment was proposed, i.e., Window 3 (w(T.Ca)=0.001 8%～0.002 8%), and the effects of  T.O, T.S, and Al_t in the steel on this window of inclusions modification were analyzed in detail and compared with the previous window of inclusions modification, and it was found that the content of T.O in the steel should be reduced as much as possible before the Ca treatment, the while ensuring the effect of Ca treatment, it can also effectively reduce the cost.

Key words: Q345R steel, inclusions, Ca treatment, reoxidation, cooling and solidification