Manufacturing of 9CrMoCoB Steel of Large Ingot with Homogeneity by ESR Process

In case of 9CrMoCoB (COST FB2) steel, equilibrium relation between [B]/[Si] ratio and (B₂O₃)/(SiO₂) ratio is very important to control [Si] and [B] in optimum range. Therefore, in this work, to investigate the thermodynamic equilibrium relation between [B]/[Si] ratio and (B₂O₃)/(SiO₂) ratio, pilot ESR experiments of 9CrMoCoB steel were carried out using the CaF₂-CaO-Al₂O₃-SiO₂-B₂O₃ slag system according to change of Si content in electrode and B₂O₃ content in the slag. Furthermore, through the test melting of the 20ton-class ESR ingot, the merits and demerits of soft arcing were investigated. From these results, it is concluded that oxygen content in the ESR ingot decrease with decreasing SiO₂ content in the slag, relation function between [B]/[Si] ratio and (B₂O₃)/(SiO₂) ratio derived by Pilot ESR test shows a good agreement as compared to the calculated line with a same slope and soft arcing makes interior and surface quality of ingot worse. With the optimized ESR conditions obtained from the present study, a 1000mm diameter (20 tons) and 2200mm diameter (120ton) 9CrMoCoB steel of the ESR ingot were successfully manufactured with good homogeneity by the ESR process. (paper)