Oxidation behavior of boron in 9CrMoCoB steel by CaF2–CaO–Al2O3–SiO2–B2O3 electroslag remelting (ESR) type slag

The laboratory-scale experiments between the CaF2–CaO–Al2O3–SiO2–B2O3 slag and 9CrMoCoB steel were carried out in alumina and magnesia crucibles at 1823 K to investigate the oxidation behavior of boron (B) during the electroslag remelting (ESR) process. The activities of SiO2 and B2O3 in the slag and the activities of Si and B in the molten steel were calculated by the ion and molecule coexistence theory (IMCT) and the Wagner formalism, respectively. The results showed that both SiO2 and B2O3 have a significant influence on the equilibrium B content. The calculated content of B was in good agreement with the experimental value when the SiO2+B2O3 content in the slag is more than 3.3 wt%. The temperature had little influence on the equilibrium B content when the SiO2, B2O3 and CaO content were in the ranges of 3–5 wt%, 0–1 wt%, and 20–30 wt%, respectively. However, the Si is more prone to oxidation than is B as the temperature increases, indicating that more SiO2 should be added in the slag to reduce the oxidation of Si. From the 80 tonnes industrial tests, the distribution of B and Si content along with the radial direction of the remelted ingot was almost uniform, which is in line with the calculated B content (approx. 30 ppm) under conditions of the Si content in the liquid steel and the (%B2O3)/(%SiO2) ratio are 0.07% and 0.05, respectively, at 1973 K.