Inclusion variations and calcium treatment optimization in pipeline steel production

SiCa line and SiCaBaFe alloy were injected into liquid pipeline steel at the end of LF refining as calcium treatment, and samples were taken from the ladles, mould, and slabs. Analysis of Ca content and inclusions shows that Ca content in steel decreases obviously in the following process after calcium treatment; the compositions, morphology, and sizes of inclusions also vary much in the production; primary inclusions in the ladles prior to calcium treatment are mainly Al(2)O(3) inclusions, but they turn to fine irregular CaS-CaO-Al(2)O(3) compound inclusions after the treatment, then become fine globular CaO-Al(2)O(3) inclusions in the mould, and finally change to a few larger irregular CaS-CaO-Al(2)O(3) complex inclusions in the slabs. Thermodynamic study reveals that inclusion variations are related with the preferential reactions among Ca, Al(2)O(3), and S and the precipitation of S in CaO-Al(2)O(3) inclusions with high sulfur capacity. New evaluation standards for calcium treatment in high-grade pipeline steel were put forward according to the inclusion variations and requirements of pipeline steel on inclusion controlling, and the calcium process was studied and optimized.