Effect of Calcium Treatment on Inclusions and Tensile Strength of Al‐Killed Steels

The effect of calcium treatment on inclusions and tensile strength of Al‐killed steels is studied through experiments and thermodynamic calculations. The addition of calcium in steel first modifies Al 2 O 3 inclusions to liquid CaO–Al 2 O 3 . The excessive calcium addition results in the formation of CaS inclusions. The solidus and liquidus of inclusions in steel first decrease and then increase with the increase of total calcium (T.Ca) content in steel, which reach the minimum value when the T.Ca content in steel is 22 ppm. With the increase of T.Ca content in steel from 0 to 70 ppm, the evolution route of inclusions is Al 2 O 3 → CaMg 2 Al 16 O 27 → CaAl 4 O 7 → liquid CaO–Al 2 O 3 → CaO–CaS. Contents of Al 2 O 3 and MgO slightly decrease with the lower temperature. The CaO gradually changes to CaS during solidification and cooling processes. The tensile strength of steel samples decreases from 809 to 737 N·mm −2 with the increase of T.Ca content in steel from 13 to 22 ppm. With a higher T.Ca content in steel, the tensile strength of steel samples increases to 774 ppm. The formation of liquid CaO–Al 2 O 3 inclusions in steel lowered the tensile strength of steel, which should be avoided during the calcium treatment process.