Investigations of Inclusion Modification in High Grade Pipeline Steel

SiCa line was injected into liquid pipeline steel at the end of LF refining as calcium treatment, and samples were taken from liquid steel in ladles before and after calcium treatment, liquid steel in mould, and slabs. Results show that Ca content in steel decreased obviously after calcium treatment with production, and the compositions, morphologies and sizes of inclusions in steel also varied with time; Al(2)O(3) inclusions turned to be fine irregular CaS-CaO-Al(2)O(3) compound inclusions after calcium treatment, and then to be fine globular CaO-Al(2)O(3) inclusions in mould, and finally changed to be a few larger irregular CaS-CaO-Al(2)O(3) complex inclusions in slabs; reoxidation of molten steel treated by Ca deteriorates effect of calcium treatment. Thermodynamic study revealed that inclusion variations were related with preferential reactions among Ca, Al(2)O(3) and S and precipitation of S in high sulfur capacity CaO-Al(2)O(3) inclusions. New calcium treatment controlling rules were put forward according to the inclusion variations and requirements of pipeline on inclusion controlling.