Evaluation standard of calcium treatment in high grade pipeline steel

Calcium treatment experiments were carried out on six heats of X70 pipeline steel.The results show that the Ca content of the steel dropped a lot and the composition and morphology of inclusions varied obviously in producing the steel after calcium treatment.Reoxidation deteriorates the effect of calcium treatment,but dissolved calcium and CaO-Al₂O₃ compound inclusions with high CaO content can play a role of modification on Al₂O₃ caused by reoxidation.The calcium treatment effect of high grade pipeline steel is related with the concentrations of calcium,oxygen and sulphur,reoxidation degree,and time after treatment,but evaluating calcium treatment with calcium content,[%Ca]_Tot/[%Al]_s,[%Ca]_Tot/T[O],ACR and [%Ca]_Tot/[%S] is improper.A evaluation standard is put forward as the following:(1) pure MnS inclusions do not exist in the central area of slabs or rolled plates,(2) n_CaO/Al2O3 of inclusions in the tundish and mould is controlled around that of 12CaO·7Al₂O₃,and (3) n_CaO/Al2O3 of inclusions after calcium treatment is higher than that of 12CaO·7Al₂O₃.It is also be careful that calcium treatment is carried out at the end of the final refining procedure so as to prevent reoxidation of liquid steel.