Abstract: In the Ti-substituted Mn process of Q355 steel, large Al₂O₃-TiO_x inclusions formed immediately after ferrotitanium addition but disappeared in later refining stages. Thermodynamic calculations and steel sample analysis show that after the addition of ferrotitanium, the local Ti concentration in the steel increases rapidly, generating Al₂O₃-TiO_x inclusions, which are then reduced by dissolved aluminum. Liquid Al-Ti-O inclusions are formed only when the Ti content in the steel reaches 0.42%. Currently, the Ti content in the steel mainly exists in the form of dissolved Ti. The Ti element causes the range of the liquid window for Ca treatment to narrow. Excessive Ca content generates solid 2CaO·Ti₂O₃ and CaS. Under the condition of ensuring reasonable calcium treatment, the evolution path of inclusions throughout the refining process is Al₂O₃-(MgO)→Al₂O₃-TiO_x→Al₂O₃-(TiO_x)→Al₂O₃-(MgO)-CaO-CaS-(TiO_x). In this experiment, the Ti-substituted Mn process reduced the cost of ferromanganese by  5.70 yuan/t, increased the cost of ferrotitanium by  1.40  yuan/t, and effectively reduce the production cost of Q355 steel by  4.30 yuan/t.

Key words: Q355 steel, inclusions, secondary refining, Ti element, Ca treatment