Effect of Na2O Addition on Crystallization Behavior and Structure of CaO–SiO2–Al2O3 Inclusions in Si–Mn Deoxidized Steel
摘要
The CaO–SiO2–Al2O3 inclusions in Si–Mn deoxidized steel tend to crystallize during the steelmaking process. The effect of Na2O addition on the crystallization behavior of CaO–SiO2–Al2O3 inclusions is studied in the current work. The continuous cooling transformation (CCT) diagrams of synthetic inclusions with different Na2O contents are constructed through single hot thermocouple technology (SHTT) experiments. The results show that the crystallization ability of low melting point CaO–SiO2–Al2O3 inclusions is enhanced with increasing the Na2O content from 0 to 12 wt pct. And the crystallization behavior of the inclusions is affected by the cooling rate. During the solidification process, the crystallization proportion of the synthetic inclusions decreases with increasing the cooling rate from 10 °C/min to 120 °C/min. However, when the Na2O content increases to above 8 wt pct, synthetic inclusions can undergo a large degree of crystallization in the cooling rate range of 10–120 °C/min. In addition, with increasing the Na2O content from 0 to 12 wt pct, the CaO activity increases from 1.49 × 10−3 to 7.08 × 10−3, the Na2O activity increases from 0 to 1.08 × 10−6, the SiO2 activity decreases from 8.81 × 10−2 to 6.89 × 10−3, and the Al2O3 activity decreases from 1.56 × 10−2 to 2.83 × 10−3, which leads to the crystalline phase in synthetic inclusions changing from Ca2Al2SiO7 to NaCaAlSi2O7 and finally to Ca2SiO4. The enhancement of crystallization ability is mainly related to the change in the melt structure. The addition of Na2O increases the depolymerization degree of the melt structure and decreases the viscosity of the inclusion melt. The resistance of crystal nucleation and growth decreases, so the crystallization ability of inclusions increases.